packaging guide: Add steps for creating missing debian/RELEASE branch

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

content/guides/apertis_packaging_guide.md

@@ -128,6 +128,12 @@ and selecting the `debian/bullseye` branch as the reference.
 
 ![Run Pipeline button](/images/run-pipeline-button.png)
 
+If the needed `debian/$RELEASE` branch doesn't exist (for example, there is a
+`debian/buster` branch but no `debian/bullseye`), the Gitlab Web UI can be used
+to create it. In the `Create from` field of the branch creation page, users
+should select `debian/${RELEASE-1}`. In this case, the new branch named
+`debian/bullseye` will be created from `debian/buster`.
+
 Reviewers can then force-push to the proposed update branch in the Merge
 Request to fixup any issue caused by the automated merge, and ultimately land
 the MRs to the `apertis/*` branches.