add doc section about --readonly option
add doc section about --readonly option

[docs only]

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