Add information about custom roles in Admin namespace.
Add information about custom roles in Admin namespace.

- Explain to user how to authorize specific admin controllers with
CanCan

- Explain to user the importance of "model_class" when defining custom
controllers in the Admin namespace.

Fixes #143

zamiang/spree,Boomkat/spree,vinsol/spree,joanblake/spree,berkes/spree,alejandromangione/spree,reinaris/spree,vinsol/spree,thogg4/spree,hifly/spree,vulk/spree,reinaris/spree,camelmasa/spree,jasonfb/spree,NerdsvilleCEO/spree,shaywood2/spree,TimurTarasenko/spree,sideci-sample/sideci-sample-spree,vcavallo/spree,NerdsvilleCEO/spree,madetech/spree,piousbox/spree,TimurTarasenko/spree,TimurTarasenko/spree,radarseesradar/spree,kewaunited/spree,gregoryrikson/spree-sample,beni55/spree,vmatekole/spree,ayb/spree,KMikhaylovCTG/spree,pulkit21/spree,jeffboulet/spree,moneyspyder/spree,tancnle/spree,gautamsawhney/spree,volpejoaquin/spree,assembledbrands/spree,abhishekjain16/spree,njerrywerry/spree,lsirivong/solidus,calvinl/spree,grzlus/solidus,agient/agientstorefront,scottcrawford03/solidus,maybii/spree,useiichi/spree,Senjai/solidus,CJMrozek/spree,jsurdilla/solidus,abhishekjain16/spree,patdec/spree,keatonrow/spree,Machpowersystems/spree_mach,jimblesm/spree,pervino/solidus,useiichi/spree,ayb/spree,dafontaine/spree,Hawaiideveloper/shoppingcart,yushine/spree,bricesanchez/spree,delphsoft/spree-store-ballchair,firman/spree,sfcgeorge/spree,SadTreeFriends/spree,vulk/spree,abhishekjain16/spree,Arpsara/solidus,jordan-brough/solidus,pulkit21/spree,adaddeo/spree,priyank-gupta/spree,calvinl/spree,net2b/spree,lsirivong/spree,CiscoCloud/spree,softr8/spree,vmatekole/spree,pulkit21/spree,jspizziri/spree,maybii/spree,lsirivong/solidus,jordan-brough/solidus,JuandGirald/spree,radarseesradar/spree,zamiang/spree,Ropeney/spree,omarsar/spree,groundctrl/spree,pervino/solidus,Hawaiideveloper/shoppingcart,TrialGuides/spree,piousbox/spree,siddharth28/spree,gregoryrikson/spree-sample,FadliKun/spree,yushine/spree,reinaris/spree,caiqinghua/spree,firman/spree,quentinuys/spree,Migweld/spree,athal7/solidus,forkata/solidus,builtbybuffalo/spree,raow/spree,grzlus/solidus,gautamsawhney/spree,pervino/spree,bonobos/solidus,karlitxo/spree,delphsoft/spree-store-ballchair,bonobos/solidus,vinayvinsol/spree,grzlus/solidus,berkes/spree,cutefrank/spree,azranel/spree,forkata/solidus,Ropeney/spree,scottcrawford03/solidus,tomash/spree,keatonrow/spree,berkes/spree,JDutil/spree,jimblesm/spree,tesserakt/clean_spree,siddharth28/spree,Senjai/solidus,surfdome/spree,joanblake/spree,caiqinghua/spree,scottcrawford03/solidus,azclick/spree,athal7/solidus,xuewenfei/solidus,DarkoP/spree,dafontaine/spree,AgilTec/spree,reidblomquist/spree,raow/spree,net2b/spree,ramkumar-kr/spree,progsri/spree,moneyspyder/spree,agient/agientstorefront,groundctrl/spree,Boomkat/spree,calvinl/spree,pulkit21/spree,wolfieorama/spree,APohio/spree,gautamsawhney/spree,xuewenfei/solidus,vinsol/spree,reidblomquist/spree,forkata/solidus,groundctrl/spree,quentinuys/spree,quentinuys/spree,Engeltj/spree,SadTreeFriends/spree,imella/spree,lsirivong/solidus,assembledbrands/spree,jspizziri/spree,calvinl/spree,grzlus/spree,net2b/spree,lsirivong/spree,firman/spree,wolfieorama/spree,sfcgeorge/spree,edgward/spree,jeffboulet/spree,TrialGuides/spree,rbngzlv/spree,carlesjove/spree,jaspreet21anand/spree,Engeltj/spree,AgilTec/spree,delphsoft/spree-store-ballchair,welitonfreitas/spree,hoanghiep90/spree,vcavallo/spree,ayb/spree,maybii/spree,odk211/spree,priyank-gupta/spree,shaywood2/spree,Hawaiideveloper/shoppingcart,Lostmyname/spree,agient/agientstorefront,builtbybuffalo/spree,bonobos/solidus,rbngzlv/spree,AgilTec/spree,jsurdilla/solidus,kewaunited/spree,lsirivong/spree,richardnuno/solidus,ramkumar-kr/spree,hoanghiep90/spree,lsirivong/solidus,degica/spree,moneyspyder/spree,carlesjove/spree,mindvolt/spree,xuewenfei/solidus,ujai/spree,rakibulislam/spree,degica/spree,fahidnasir/spree,Machpowersystems/spree_mach,useiichi/spree,alejandromangione/spree,FadliKun/spree,ujai/spree,alejandromangione/spree,yiqing95/spree,nooysters/spree,edgward/spree,madetech/spree,DynamoMTL/spree,Migweld/spree,bonobos/solidus,miyazawatomoka/spree,orenf/spree,scottcrawford03/solidus,volpejoaquin/spree,omarsar/spree,edgward/spree,robodisco/spree,edgward/spree,piousbox/spree,Ropeney/spree,beni55/spree,priyank-gupta/spree,yushine/spree,jasonfb/spree,Kagetsuki/spree,dandanwei/spree,delphsoft/spree-store-ballchair,shekibobo/spree,karlitxo/spree,APohio/spree,softr8/spree,bricesanchez/spree,vcavallo/spree,progsri/spree,ujai/spree,thogg4/spree,raow/spree,devilcoders/solidus,shaywood2/spree,grzlus/spree,locomotivapro/spree,vinayvinsol/spree,tomash/spree,jparr/spree,Kagetsuki/spree,vmatekole/spree,Arpsara/solidus,alvinjean/spree,fahidnasir/spree,ahmetabdi/spree,Hawaiideveloper/shoppingcart,rakibulislam/spree,trigrass2/spree,maybii/spree,CiscoCloud/spree,nooysters/spree,quentinuys/spree,Machpowersystems/spree_mach,welitonfreitas/spree,zaeznet/spree,miyazawatomoka/spree,grzlus/spree,brchristian/spree,progsri/spree,tancnle/spree,locomotivapro/spree,pervino/solidus,Boomkat/spree,sliaquat/spree,zamiang/spree,moneyspyder/spree,karlitxo/spree,wolfieorama/spree,fahidnasir/spree,azclick/spree,locomotivapro/spree,brchristian/spree,rajeevriitm/spree,vmatekole/spree,TrialGuides/spree,cutefrank/spree,odk211/spree,gregoryrikson/spree-sample,jparr/spree,raow/spree,shekibobo/spree,alejandromangione/spree,joanblake/spree,NerdsvilleCEO/spree,kewaunited/spree,DynamoMTL/spree,vulk/spree,yushine/spree,imella/spree,sfcgeorge/spree,rbngzlv/spree,FadliKun/spree,ckk-scratch/solidus,vinayvinsol/spree,rbngzlv/spree,reidblomquist/spree,lsirivong/spree,KMikhaylovCTG/spree,azranel/spree,richardnuno/solidus,njerrywerry/spree,vcavallo/spree,camelmasa/spree,APohio/spree,ckk-scratch/solidus,karlitxo/spree,TimurTarasenko/spree,omarsar/spree,jaspreet21anand/spree,piousbox/spree,rajeevriitm/spree,Engeltj/spree,jasonfb/spree,zamiang/spree,hifly/spree,caiqinghua/spree,zaeznet/spree,project-eutopia/spree,Arpsara/solidus,AgilTec/spree,tesserakt/clean_spree,Migweld/spree,vulk/spree,ramkumar-kr/spree,lyzxsc/spree,DarkoP/spree,athal7/solidus,welitonfreitas/spree,adaddeo/spree,Lostmyname/spree,adaddeo/spree,CJMrozek/spree,degica/spree,Lostmyname/spree,project-eutopia/spree,softr8/spree,abhishekjain16/spree,jeffboulet/spree,rajeevriitm/spree,keatonrow/spree,shekibobo/spree,jparr/spree,madetech/spree,tesserakt/clean_spree,patdec/spree,archSeer/spree,jparr/spree,grzlus/spree,Migweld/spree,athal7/solidus,CJMrozek/spree,mleglise/spree,KMikhaylovCTG/spree,gautamsawhney/spree,miyazawatomoka/spree,brchristian/spree,jimblesm/spree,hoanghiep90/spree,mleglise/spree,trigrass2/spree,locomotivapro/spree,keatonrow/spree,progsri/spree,omarsar/spree,SadTreeFriends/spree,sunny2601/spree,surfdome/spree,reidblomquist/spree,vinsol/spree,jaspreet21anand/spree,azranel/spree,useiichi/spree,CiscoCloud/spree,Arpsara/solidus,firman/spree,kewaunited/spree,odk211/spree,tancnle/spree,orenf/spree,thogg4/spree,zaeznet/spree,derekluo/spree,JDutil/spree,ramkumar-kr/spree,alvinjean/spree,mindvolt/spree,ahmetabdi/spree,patdec/spree,carlesjove/spree,Kagetsuki/spree,sunny2601/spree,beni55/spree,assembledbrands/spree,derekluo/spree,PhoenixTeam/spree_phoenix,vinayvinsol/spree,net2b/spree,azclick/spree,PhoenixTeam/spree_phoenix,PhoenixTeam/spree_phoenix,camelmasa/spree,devilcoders/solidus,jeffboulet/spree,jaspreet21anand/spree,madetech/spree,jsurdilla/solidus,cutefrank/spree,imella/spree,volpejoaquin/spree,JuandGirald/spree,Lostmyname/spree,DynamoMTL/spree,Boomkat/spree,tomash/spree,welitonfreitas/spree,caiqinghua/spree,bricesanchez/spree,sfcgeorge/spree,miyazawatomoka/spree,grzlus/solidus,adaddeo/spree,tancnle/spree,mindvolt/spree,njerrywerry/spree,tesserakt/clean_spree,derekluo/spree,gregoryrikson/spree-sample,pervino/solidus,Ropeney/spree,dandanwei/spree,sunny2601/spree,robodisco/spree,CiscoCloud/spree,devilcoders/solidus,robodisco/spree,berkes/spree,fahidnasir/spree,sliaquat/spree,mindvolt/spree,ckk-scratch/solidus,shekibobo/spree,jasonfb/spree,project-eutopia/spree,siddharth28/spree,Engeltj/spree,alvinjean/spree,dandanwei/spree,SadTreeFriends/spree,ahmetabdi/spree,TrialGuides/spree,azranel/spree,project-eutopia/spree,shaywood2/spree,JDutil/spree,pervino/spree,JuandGirald/spree,trigrass2/spree,xuewenfei/solidus,carlesjove/spree,odk211/spree,tomash/spree,groundctrl/spree,alvinjean/spree,richardnuno/solidus,reinaris/spree,mleglise/spree,derekluo/spree,jspizziri/spree,joanblake/spree,jordan-brough/solidus,devilcoders/solidus,pervino/spree,nooysters/spree,lyzxsc/spree,patdec/spree,rajeevriitm/spree,jordan-brough/solidus,forkata/solidus,siddharth28/spree,NerdsvilleCEO/spree,ahmetabdi/spree,radarseesradar/spree,jimblesm/spree,trigrass2/spree,sliaquat/spree,camelmasa/spree,zaeznet/spree,DynamoMTL/spree,orenf/spree,builtbybuffalo/spree,PhoenixTeam/spree_phoenix,JuandGirald/spree,jspizziri/spree,hifly/spree,rakibulislam/spree,KMikhaylovCTG/spree,dafontaine/spree,thogg4/spree,cutefrank/spree,softr8/spree,nooysters/spree,azclick/spree,DarkoP/spree,dafontaine/spree,hoanghiep90/spree,Kagetsuki/spree,yiqing95/spree,beni55/spree,rakibulislam/spree,sunny2601/spree,richardnuno/solidus,lyzxsc/spree,JDutil/spree,sideci-sample/sideci-sample-spree,Senjai/solidus,sliaquat/spree,volpejoaquin/spree,hifly/spree,wolfieorama/spree,orenf/spree,surfdome/spree,jsurdilla/solidus,archSeer/spree,ayb/spree,pervino/spree,robodisco/spree,archSeer/spree,builtbybuffalo/spree,yiqing95/spree,lyzxsc/spree,archSeer/spree,radarseesradar/spree,mleglise/spree,sideci-sample/sideci-sample-spree,njerrywerry/spree,Senjai/solidus,CJMrozek/spree,FadliKun/spree,surfdome/spree,brchristian/spree,ckk-scratch/solidus,priyank-gupta/spree,DarkoP/spree,agient/agientstorefront,yiqing95/spree,dandanwei/spree,APohio/spree