apertis_packaging_guide: Mention the child pipeline for custom jobs

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/guides/apertis_packaging_guide.md

@@ -417,6 +417,18 @@ under `debian/apertis/copyright`, updating the merge request when necessary.
 [Dpkg::Copyright::Scanner]: https://manpages.debian.org/testing/libconfig-model-dpkg-perl/Dpkg::Copyright::Scanner.3pm.en.html
 [gitignore]: https://manpages.debian.org/testing/git-man/gitignore.5.en.html
 
+## Custom pipelines
+
+When using the packaging pipeline, developers cannot put their CI/CD automation
+in `.gitlab-ci.yml` anymore, as the CI config path points to the
+ci-package-builder definition.
+
+However, developers can put their jobs in the
+`debian/apertis/local-gitlab-ci.yml` file and have them executed in a child
+pipeline whenever the main packaging pipeline is executed. This is specially
+handy to run tests before the actual packaging process begins.
+
+
 # Internals
 
 Main components: