Add whatsnew docs for 3.1 cosmology.
Add whatsnew docs for 3.1 cosmology.

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