Update README.md with API key guidance.
Update README.md with API key guidance.

shahawi/Sunshine-Version-2,brdahlgren/Sunshine,amit-sgwn/Sunshine-Version-2,udacity/Sunshine-Version-2,arcadeperfect/mySunshine,ramanareddysane/Sunshine-Version-2,bearbear24/sunshine,KarenWest/Sunshine-Version-2,loko087/Sunshine-Version-2,kimcy12/Notifier,BabaratusCorp/babateo,kimcy12/Lemonade-LME,taimur97/Sunshine-Version-2,aorura/TakeAUmbrella,Bhavana1801/weatherApp,ashleysimpson/sunshine,DkPhoenix/Sunshine-Version-2,opklnm102/Sunshine-Version-2,janzoner/Sunshine-Version-2