Add more information about subclassing in nddata
Add more information about subclassing in nddata

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