Add finite difference description
Add finite difference description

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