Add concise description of rebasing to contributor workflow
Add concise description of rebasing to contributor workflow

Closes #333

MSeifert04/astropy,StuartLittlefair/astropy,stargaser/astropy,stargaser/astropy,astropy/astropy,DougBurke/astropy,kelle/astropy,mhvk/astropy,StuartLittlefair/astropy,tbabej/astropy,astropy/astropy,joergdietrich/astropy,astropy/astropy,stargaser/astropy,astropy/astropy,funbaker/astropy,DougBurke/astropy,dhomeier/astropy,kelle/astropy,funbaker/astropy,funbaker/astropy,aleksandr-bakanov/astropy,tbabej/astropy,pllim/astropy,kelle/astropy,DougBurke/astropy,joergdietrich/astropy,mhvk/astropy,stargaser/astropy,pllim/astropy,lpsinger/astropy,bsipocz/astropy,AustereCuriosity/astropy,larrybradley/astropy,bsipocz/astropy,mhvk/astropy,astropy/astropy,AustereCuriosity/astropy,funbaker/astropy,AustereCuriosity/astropy,lpsinger/astropy,joergdietrich/astropy,tbabej/astropy,AustereCuriosity/astropy,larrybradley/astropy,joergdietrich/astropy,dhomeier/astropy,saimn/astropy,MSeifert04/astropy,DougBurke/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,lpsinger/astropy,larrybradley/astropy,lpsinger/astropy,MSeifert04/astropy,tbabej/astropy,StuartLittlefair/astropy,saimn/astropy,kelle/astropy,lpsinger/astropy,MSeifert04/astropy,saimn/astropy,StuartLittlefair/astropy,joergdietrich/astropy,dhomeier/astropy,dhomeier/astropy,larrybradley/astropy,AustereCuriosity/astropy,aleksandr-bakanov/astropy,bsipocz/astropy,mhvk/astropy,saimn/astropy,saimn/astropy,larrybradley/astropy,bsipocz/astropy,pllim/astropy,tbabej/astropy,pllim/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,mhvk/astropy,kelle/astropy,pllim/astropy