CONTRIBUTING: Clarify some constraints for PR submission.

This commit updates the contribution guidelines to clarify some points
related to submitting PRs in a way that wouldn't fail basic CI checks.

The contribution guidelines assumed that all PRs would come from a
properly set-up environment, which it may not always be the case for
small changes being done from within GitHub itself.  However these PRs
would not undergo the checks provided by the pre-commit hooks that are
set up in a properly set-up environment.

These changes aim to dissuade users who read the guidelines first to
submit PRs from GitHub directly but to do so from a configured
environment, and if that is not the case to at least make explicit the
commit message validation criteria.  There isn't much to do for code
formatting rules besides saying to use "ruff" first, unfortunately.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

CONTRIBUTING.md

@@ -37,6 +37,13 @@ Similar to the main repository, a configuration is provided for the
 rules and apply linting checks. See the documentation for using pre-commit
 in [the code conventions document](https://github.com/micropython/micropython/blob/master/CODECONVENTIONS.md#automatic-pre-commit-hooks)
 
+While it is possible to create simple PRs from within GitHub itself, doing so
+is not really recommended. Doing so would bypass the pre-commit checks and
+you will end up with a PR that will potentially fail one or more CI jobs due
+to a commit subject/message that does not follow the format conventions, code
+that was not formatted using `ruff` beforehand, or for simpler things like a
+typo or a spelling error.
+
 In addition to the conventions from the main repository, there are some
 specific conventions and guidelines for micropython-lib: