Add basic docs on renewal (fixes #951).
Add basic docs on renewal (fixes #951).

xgin/letsencrypt,mrb/letsencrypt,jtl999/certbot,lmcro/letsencrypt,lmcro/letsencrypt,brentdax/letsencrypt,hsduk/lets-encrypt-preview,dietsche/letsencrypt,jsha/letsencrypt,thanatos/lets-encrypt-preview,Sveder/letsencrypt,letsencrypt/letsencrypt,piru/letsencrypt,xgin/letsencrypt,ghyde/letsencrypt,jtl999/certbot,mitnk/letsencrypt,stweil/letsencrypt,mrb/letsencrypt,DavidGarciaCat/letsencrypt,twstrike/le_for_patching,jsha/letsencrypt,martindale/letsencrypt,bsmr-misc-forks/letsencrypt,Sveder/letsencrypt,hsduk/lets-encrypt-preview,wteiken/letsencrypt,sjerdo/letsencrypt,thanatos/lets-encrypt-preview,mitnk/letsencrypt,piru/letsencrypt,kuba/letsencrypt,lbeltrame/letsencrypt,kuba/letsencrypt,bsmr-misc-forks/letsencrypt,twstrike/le_for_patching,wteiken/letsencrypt,VladimirTyrin/letsencrypt,letsencrypt/letsencrypt,ghyde/letsencrypt,TheBoegl/letsencrypt,TheBoegl/letsencrypt,BillKeenan/lets-encrypt-preview,BillKeenan/lets-encrypt-preview,dietsche/letsencrypt,sjerdo/letsencrypt,martindale/letsencrypt,brentdax/letsencrypt,lbeltrame/letsencrypt,stweil/letsencrypt,goofwear/letsencrypt,DavidGarciaCat/letsencrypt,VladimirTyrin/letsencrypt,goofwear/letsencrypt