Add documentation about open file testing.
Add documentation about open file testing.

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