Added documentation for Jekyll environment variables
Added documentation for Jekyll environment variables

Information about Jekyll environment variables was missing from the documentation. This seemed like a good place to include it.
jmepg/jekyll,LeuisKen/jekyllcn,noikiy/jekyll,CPVPLachlan/CPVP-Blog,superve/jekyll,nasht00/jekyll,brint/jekyll,jmepg/jekyll,jekyll/jekyll,princeofdarkness76/jekyll,brint/jekyll,rebornix/jekyll,noikiy/jekyll,chinayin/jekyll,princeofdarkness76/jekyll,jaybe-jekyll/jekyll,derekgottlieb/jekyll,gencer/jekyll,ducktyper/jekyll,Kinghack/kinghack.github.com,nickg33/jekyll,chinayin/jekyll,vidbina/jekyll,x-way/jekyll,FlyingWHR/jekyll,pmarsceill/jekyll,aarvay/jekyll,xiebinhqy/jklly_china,bsmr-ruby/jekyll,jeffkole/jekyll,x-way/jekyll,CPVPLachlan/CPVP-Blog,aarvay/jekyll,mrb/jekyll,mrb/jekyll,pmarsceill/jekyll,tomjohnson1492/jekyll,xcatliu/jekyllcn,codeclimate-testing/jekyll,tomasdiez/jekyll,shliujing/jekyll,chrisfinazzo/jekyll,superve/jekyll,thetaxman/jekyll,gaosboy/jekyll,dansef/jekyll,tillgrallert/jekyll,ajhit406/jekyll,darwin/jekyll,fulldecent/jekyll,ryanshaw/jekyll,ajhit406/jekyll,ZDroid/jekyll,rlugojr/jekyll,darwin/jekyll,nasht00/jekyll,thetaxman/jekyll,ducktyper/jekyll,larryfox/jekyll,sanxore/sanxore.github.com,superve/jekyll,yihangho/jekyll,mrb/jekyll,MauroMrod/jekyll,xiebinhqy/jklly_china,tomasdiez/jekyll,gencer/jekyll,chinayin/jekyll,chrisfinazzo/jekyll,anthonyrosengren/grasshoppergathering2.github-io,xiebinhqy/jklly_china,shliujing/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,CPVPLachlan/CPVP-Blog,CheekyFE/jekyll,yihangho/jekyll,mrb/jekyll,getandpost/jekyll.github.io,darwin/jekyll,tomjohnson1492/jekyll,sasongkojati/jekyll,Kinghack/kinghack.github.com,stomar/jekyll,likong/jekyll,likong/jekyll,dansef/jekyll,ryanshaw/jekyll,rlugojr/jekyll,chrisfinazzo/jekyll,derekgottlieb/jekyll,hartmel/jekyll,floydpraveen/jekyll,thetaxman/jekyll,floydpraveen/jekyll,Strangehill/jekyll,nickg33/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,LeuisKen/jekyllcn,rasa2011/rasa2011.github.io,Kinghack/kinghack.github.com,anthonyrosengren/grasshoppergathering2.github-io,Strangehill/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,jaybe-jekyll/jekyll,howkj1/jekyll,getandpost/jekyll.github.io,Strangehill/jekyll,getandpost/jekyll.github.io,digideskio/jekyll,CheekyFE/jekyll,rlugojr/jekyll,anthonyrosengren/grasshoppergathering2.github-io,digideskio/jekyll,ajhit406/jekyll,zsyed91/jekyll,tillgrallert/jekyll,FlyingWHR/jekyll,gencer/jekyll,alex-kovac/jekyll,ZDroid/jekyll,aarvay/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,rlugojr/jekyll,likong/jekyll,zsyed91/jekyll,FlyingWHR/jekyll,tillgrallert/jekyll,hartmel/jekyll,alfredxing/jekyll,xiebinhqy/jklly_china,shliujing/jekyll,alex-kovac/jekyll,jeffkole/jekyll,alfredxing/jekyll,derekgottlieb/jekyll,floydpraveen/jekyll,fulldecent/jekyll,MauroMrod/jekyll,Teino1978-Corp/jekyll,alfredxing/jekyll,PepSalehi/jekyll,nasht00/jekyll,xcatliu/jekyllcn,sanxore/sanxore.github.com,xcatliu/jekyllcn,kuangyeheng/jekyll,sasongkojati/jekyll,bsmr-ruby/jekyll,tomjohnson1492/jekyll,larryfox/jekyll,brint/jekyll,digideskio/jekyll,zsyed91/jekyll,Strangehill/jekyll,nickg33/jekyll,zsyed91/jekyll,tomjohnson1492/jekyll,rasa2011/rasa2011.github.io,ajhit406/jekyll,brint/jekyll,noikiy/jekyll,ryanshaw/jekyll,PepSalehi/jekyll,vidbina/jekyll,princeofdarkness76/jekyll,stomar/jekyll,stomar/jekyll,gencer/jekyll,gaosboy/jekyll,yihangho/jekyll,dansef/jekyll,pheuko/jekyll,Teino1978-Corp/jekyll,rebornix/jekyll,fulldecent/jekyll,alex-kovac/jekyll,tomasdiez/jekyll,alex-kovac/jekyll,x-way/jekyll,jeffkole/jekyll,pmarsceill/jekyll,jeffkole/jekyll,gencer/jekyll,kuangyeheng/jekyll,derekgottlieb/jekyll,codeclimate-testing/jekyll,alex-kovac/jekyll,PepSalehi/jekyll,pheuko/jekyll,gaosboy/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,LeuisKen/jekyllcn,sasongkojati/jekyll,pmarsceill/jekyll,stomar/jekyll,alfredxing/jekyll,xcatliu/jekyllcn,jekyll/jekyll,jaybe-jekyll/jekyll,kuangyeheng/jekyll,jmepg/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,nasht00/jekyll,ducktyper/jekyll,darwin/jekyll,rasa2011/rasa2011.github.io,larryfox/jekyll,rebornix/jekyll,yhironaka/yhironaka.github.io,CheekyFE/jekyll,x-way/jekyll,bsmr-ruby/jekyll,jekyll/jekyll,ZDroid/jekyll,floydpraveen/jekyll,pheuko/jekyll,bsmr-ruby/jekyll,MauroMrod/jekyll,princeofdarkness76/jekyll,codeclimate-testing/jekyll,vidbina/jekyll,howkj1/jekyll,Strangehill/jekyll,howkj1/jekyll,hartmel/jekyll,Teino1978-Corp/jekyll,noikiy/jekyll,jekyll/jekyll,sanxore/sanxore.github.com,tomjohnson1492/jekyll