Add whatsnew for FITS performance improvements [docs only]
Add whatsnew for FITS performance improvements [docs only]

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