doc: require CI status indicator in PRs

Commits are often landed despite failing on one or more CI platforms.
Having a CI status indicator in the PR should make this less likely to
happen.

PR-URL: https://github.com/nodejs/node/pull/17151
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

COLLABORATOR_GUIDE.md

@@ -142,6 +142,7 @@ test should *fail* before the change, and *pass* after the change.
 All pull requests that modify executable code should be subjected to
 continuous integration tests on the
 [project CI server](https://ci.nodejs.org/).
+The pull request should have a CI status indicator if possible.
 
 #### Useful CI Jobs