add info about velocity support for SkyCoord in docs (and remove experimental warning)
add info about velocity support for SkyCoord in docs (and remove experimental warning)

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