add doc section on configuration
add doc section on configuration

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