added description of Galactocentric transformation to docs
added description of Galactocentric transformation to docs

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