Add comment about backwards compatibility
Add comment about backwards compatibility

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