updates to cextern/README.rst reflecting re-thought cextern structure
updates to cextern/README.rst reflecting re-thought cextern structure

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