Update note on how to build docs for a release

Author: PierreF

CONTRIBUTING.md

@@ -90,9 +90,11 @@ Making a release
 ----------------
 
 The process to make a release is the following:
+* Using a virtual env with the following tool installed: `pip install build sphinx twine`
+* In that same virtual env, install paho itself (required for docs): `pip install -e .`
 * Update the Changelog with the release version and date. Ensure it's up-to-date with latest fixes & PRs merged.
 * Make sure test pass, check that Github actions are green.
-* Check that documentation build (`cd docs; make html`)
+* Check that documentation build (`cd docs; make clean html`)
 * Bump the version number in ``paho/mqtt/__init__.py``, commit the change.
 * Make a dry-run of build:
    * Build using hatch: ``python -m hatch build``