doc: explain what to do if git push is rejected

In COLLABORATOR_GUIDE.md, explain what to do if `git push upstream
master` is rejected.

PR-URL: https://github.com/nodejs/node/pull/14848
Fixes: https://github.com/nodejs/node/issues/12628


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>