doc: ensure collaborators validate commits

This should be done every time a collaborator pushes a commit.

PR-URL: #16162
Reviewed-By: Joyee Cheung <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Roman Reiss <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>

Author: bmeck

COLLABORATOR_GUIDE.md

@@ -517,6 +517,13 @@ Run tests (`make -j4 test` or `vcbuild test`). Even though there was a
 successful continuous integration run, other changes may have landed on master
 since then, so running the tests one last time locally is a good practice.
 
+Validate that the commit message is properly formatted using
+[core-validate-commit](https://github.com/evanlucas/core-validate-commit).
+
+```text
+$ git rev-list upstream/master...HEAD | xargs core-validate-commit
+```
+
 Time to push it:
 
 ```text