doc: add pref to using draft PR versus WIP label

- add to the contribution guide that we prefer that
  people use draft PRs instead of the WIP label.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: #40824
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/guides/contributing/pull-requests.md

@@ -271,6 +271,11 @@ details, but feel free to skip parts if you're not sure what to put.
 
 Once opened, pull requests are usually reviewed within a few days.
 
+To get feedback on your proposed change even though it is not ready
+to land, use the `Convert to draft` option in the GitHub UI.
+Do not use the `wip` label as it might not prevent the PR
+from landing before you are ready.
+
 ### Step 9: Discuss and update
 
 You will probably get feedback or requests for changes to your pull request.