Merge #14600: docs: Clarify commit message guidelines

0e6de3a added details about commit messages (Martin Erlandsson)

Pull request description:

  In this small PR, the gist of [this helpful and informative comment from @fanquake](bitcoin/bitcoin#14583 (comment)) is added to the official contributing instructions, to help future first-time contributors get their commit messages right.

Tree-SHA512: ca6e9b639e007944314690ef771470f346d6f0ad8aa398abf86e45258bc617334fdbd2ca10ff930d62669a5eca5b507a3ebae11f75971f80f8a44b0bb17ab054

Author: laanwj

CONTRIBUTING.md

@@ -67,6 +67,8 @@ If a particular commit references another issue, please add the reference. For
 example: `refs #1234` or `fixes #4321`. Using the `fixes` or `closes` keywords
 will cause the corresponding issue to be closed when the pull request is merged.
 
+Commit messages should never contain any `@` mentions.
+
 Please refer to the [Git manual](https://git-scm.com/doc) for more information
 about Git.
 
@@ -135,6 +137,10 @@ before it will be merged. The basic squashing workflow is shown below.
     # Save and quit.
     git push -f # (force push to GitHub)
 
+Please update the resulting commit message if needed, it should read as a
+coherent message. In most cases this means that you should not just list the
+interim commits.
+
 If you have problems with squashing (or other workflows with `git`), you can
 alternatively enable "Allow edits from maintainers" in the right GitHub
 sidebar and ask for help in the pull request.