Update CONTRIBUTING.md
Update CONTRIBUTING.md

Asking contributors to point to `master` branch
AJIXuMuK/sp-dev-fx-webparts,AJIXuMuK/sp-dev-fx-webparts,SharePoint/sp-dev-fx-webparts,AJIXuMuK/sp-dev-fx-webparts,SharePoint/sp-dev-fx-webparts