devel-docs: some info on new Gitlab MR feature to create test packages.

How to create the packages through MR labels and where to find the
resulting test packages…

Note that it would be nice if we had automatic messages on the MR
writing down the procedure (with generated links for the specific
pipeline ID) once a pipeline succeeds.
It would simplify the whole process even further. We can see some other
day how such a thing could be automated.

devel-docs/gitlab-mr.md

@@ -0,0 +1,46 @@
+# Merge Request tricks
+
+By default, a Merge Request pipeline would only build GIMP with
+autotools, meson and for Windows 64-bit (similarly to normal commits).
+
+You might want to actually generate easy-to-install builds, in
+particular if you want it to be testable for non-developers, or various
+other reasons. Making a full flatpak or Windows installer can actually
+be quite time-consuming on a personal computer.
+
+☣️  We remind that these packages are built on-top of development code
+(i.e. work-in-progress and potentially unstable codebase likely
+containing critical bugs) with additional code which can be contributed
+by anyone (any anonymous person is allowed to propose patches as merge
+requests not only known team members).
+Therefore you should always check the merge request changes before
+running the code and never blindly trust that it is harmless. In any
+case, run these builds at your own risk. ☢️
+
+## Generating a Windows installer for merge request code
+
+If you add the label `5. Windows Installer` in a MR, then trigger a
+pipeline (for instance by rebasing), it will add a Windows installer
+creation to the pipeline. Once the pipeline ends, the installer can be
+found by:
+
+- clicking the pipeline ID.
+- In the "Distribution" stage, click the "win-installer-nightly" job.
+- Then click the "Browse" button.
+- Navigate to `build/installer/_Output/`.
+- Then click the `gimp-<version>-setup.exe` file to download the
+  installer.
+
+## Generating a flatpak for merge request code
+
+If you add the label `5. Flatpak package` in a MR, then trigger a
+pipeline for instance by rebasing), it will add a flatpak creation to
+the pipeline. Once the pipeline ends, the flatpak can be installed by:
+
+- clicking the pipeline ID.
+- In the "Gimp" stage, click the "flatpak" job.
+- Then click the "Browse" button.
+- Click the `gimp-git.flatpak` file to download it.
+- Locally run: `flatpak install --user ./gimp-git.flatpak`
+  It should propose you to install the flatpak, allowing you to test.
+- After testing, you can uninstall with: `flatpak remove org.gimp.GIMP//master`