docs: add branch & fork steps to contributing documentation
docs: add branch & fork steps to contributing documentation

closes #2881

hacksparrow/express,wesleytodd/express,strongloop/express,WebStyle/express,expressjs/express,YR/express,expressjs/express,ralic/express,hacksparrow/express,strongloop/express,YR/express,ralic/express,wesleytodd/express,WebStyle/express