Update CONTRIBUTING.md
Update CONTRIBUTING.md

A useful tip to include in the contributing guidelines.
lpsinger/astropy,dhomeier/astropy,joergdietrich/astropy,funbaker/astropy,saimn/astropy,larrybradley/astropy,funbaker/astropy,larrybradley/astropy,stargaser/astropy,DougBurke/astropy,bsipocz/astropy,kelle/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,mhvk/astropy,aleksandr-bakanov/astropy,AustereCuriosity/astropy,lpsinger/astropy,DougBurke/astropy,larrybradley/astropy,tbabej/astropy,lpsinger/astropy,pllim/astropy,kelle/astropy,AustereCuriosity/astropy,dhomeier/astropy,saimn/astropy,AustereCuriosity/astropy,stargaser/astropy,StuartLittlefair/astropy,mhvk/astropy,bsipocz/astropy,astropy/astropy,DougBurke/astropy,funbaker/astropy,pllim/astropy,stargaser/astropy,tbabej/astropy,astropy/astropy,MSeifert04/astropy,astropy/astropy,kelle/astropy,kelle/astropy,MSeifert04/astropy,joergdietrich/astropy,tbabej/astropy,StuartLittlefair/astropy,astropy/astropy,StuartLittlefair/astropy,joergdietrich/astropy,AustereCuriosity/astropy,MSeifert04/astropy,pllim/astropy,mhvk/astropy,astropy/astropy,saimn/astropy,stargaser/astropy,mhvk/astropy,DougBurke/astropy,bsipocz/astropy,lpsinger/astropy,dhomeier/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,joergdietrich/astropy,pllim/astropy,StuartLittlefair/astropy,MSeifert04/astropy,tbabej/astropy,lpsinger/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,larrybradley/astropy,joergdietrich/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,tbabej/astropy,kelle/astropy,AustereCuriosity/astropy,pllim/astropy,funbaker/astropy,saimn/astropy