README: Document import-debian-package steps

import-debian-package script now runs all the steps automatatically. The
recently added --push-remote option allows creating and pushing the
imported package to apertis remote gitlab instance.

Document the previously manual steps (performed now by the script).

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

README

@@ -6,5 +6,37 @@ then debug them.
 
 See the man page for more details about the available commands and subcommands.
 
-* `import-debian-package`: imports a package from Debian in the format
+## import-debian-package
+
+`import-debian-package` script imports a package from Debian in the format
 expected by the Apertis CI system.
+
+To import a new package from Debian the previous process required several manual
+steps which are now performed automatically by the script using the
+`--push-remote` option.
+
+The following steps are now automatized:
+* package folder is created and import process is performed in there.
+* target debian package is fetched and imported.
+* remote repo is created if it doesn't exist.
+  * Create an empty project on GitLab under the `pkg` namespace (for instance,
+    `pkg/hello`). Choose visibility level `Public` when creating the project.
+  * Configure the origin remote on your local git:
+```
+$ git remote add origin git@gitlab.apertis.org:pkg/hello
+```
+* remote branches and tags are pushed.
+```
+$ git push --all --follow-tags origin
+```
+* gitlab settings are applied to remote repo using `gitlab-rulez`:
+  * sets the CI config path to
+    `ci-package-builder.yml@infrastructure/ci-package-builder`
+  * changes the merge request settings to:
+    * only allow fast-forward merges
+    * ensure merges are only allowed if pipelines succeed
+  * marks the `apertis/*` and `debian/*` branches as protected
+```
+$ gitlab-rulez apply ../apertis-infrastructure/gitlab-scripts/rulez.yaml --filter pkg/hello
+```
+* CI pipeline is triggered on all downstream branches.