Add instructions on how to start deploy from command-line

RELEASING.rst

@@ -3,5 +3,10 @@ Here are the steps on how to make a new release.
 1. Create a ``release-VERSION`` branch from ``upstream/main``.
 2. Update ``CHANGELOG.rst``.
 3. Push the branch to ``upstream``.
-4. Once all tests pass, start the ``deploy`` workflow manually.
+4. Once all tests pass, start the ``deploy`` workflow manually or via:
+
+   ```
+   gh workflow run deploy.yml --repo pytest-dev/pytest-mock --ref release-VERSION -f version=VERSION
+   ```
+
 5. Merge the PR.