gitlab: Document how forks can use different set of jobs

Document how forks can use different set of jobs and add
a big warning so no new configuration is added to this
file.

Suggested-by: Daniel Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210525082556.4011380-4-f4bug@amsat.org>

.gitlab-ci.yml

+#
+# This is the GitLab CI configuration file for the mainstream QEMU
+# project: https://gitlab.com/qemu-project/qemu/-/pipelines
+#
+# !!! DO NOT ADD ANY NEW CONFIGURATION TO THIS FILE !!!
+#
+# Only documentation or comments is accepted.
+#
+# To use a different set of jobs than the mainstream QEMU project,
+# you need to set the location of your custom yml file at "custom CI/CD
+# configuration path", on your GitLab CI namespace:
+# https://docs.gitlab.com/ee/ci/pipelines/settings.html#custom-cicd-configuration-path
+#
 include:
   - local: '/.gitlab-ci.d/qemu-project.yml'