Add documentation to README.md

README.md

 # gitlab-ci
 
-GitLab CI scripts
+GitLab CI scripts
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+
+This repository contains scripts for Yocto infrastructure tasks, e.g. automatic
+integration of project changes into the manifest. It is meant to be included into all
+relevant Yocto repositories as a [git submodule][1].
+
+
+### Providing `GITBOT_TOKEN`
+
+The scripts need a [personal GitLab access token][2] of a user with access to all
+relevant repositories (manifest and all contained projects) in order to work correctly.
+We have created the @guf-gitbot user for this task. Norman and Tim know the login
+credentials for it. These should not be needed, though, as the user is member of the
+[Garz & Fricke][3] group and thus already has access to all group projects. However,
+everyone can add the user as a member to every project he has access to, if needed.
+
+The personal access token of the @guf-gitbot user has to be provided on each repository
+via the CI environment variable `GITBOT_TOKEN` (set under project's settings -> CI/CD
+-> Variables). Its value can be read on projects which already have this variable
+defined.
+
+
+[1]: https://git-scm.com/book/en/v2/Git-Tools-Submodules
+[2]: https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html
+[3]: https://gitlab.com/garz-fricke