Document update PR from Fork (#1606)

* Document update PR from Fork

MAINTAINING.md

@@ -0,0 +1,21 @@
+# How to Maintain LightlySSL
+
+This document is intended for maintainers of Lightly**SSL**. If you would like to
+contribute, please refer to the [CONTRIBUTING.md](./CONTRIBUTING.md) document.
+
+## Update PR from a Fork
+
+Sometimes it is necessary to update a PR from a forked Lightly**SSL** repository,
+for example to add some finishing touches (format, cleanup, docs), to fix some difficult
+issue, or to finish an incomplete feature.
+
+Let's assume there is a PR from `<username>:<branch-name>`. To update the PR, follow
+these steps:
+
+```
+git remote add <username> https://github.com/<username>/lightly.git
+git fetch <username>
+git checkout -b <branch-name> <username>/<branch-name>
+```
+
+Now you can make changes and push them to the PR branch with `git push`.