Merge pull request 'Init CONTRIBUTING.md' (#210) from docs/add-contributing into main

Reviewed-on: pub-solar/os#210
Reviewed-by: b12f <hello@benjaminbaedorf.eu>

CONTRIBUTING.md

@@ -0,0 +1,28 @@
+# Quick branch overview
+We work with several branches in this repo. This document aims to explain how
+to contribute changes to the existing branches.
+
+### `main` branch
+- Changes to `modules` and `profiles` should go [the main branch](https://git.pub.solar/pub-solar/os/src/branch/main)
+- Changes can get accepted via: Pull Request
+- Branch protected from direct `git push`
+
+### `infra` branch
+- Changes to the [pub.solar](https://pub.solar) infrastructure should be merged [into this branch](https://git.pub.solar/pub-solar/os/src/branch/infra)
+- Changes can get accepted via: Pull Request
+- Branch protected from direct `git push`
+
+### `momo/main` branch
+- Changes to the [Momo](https://momo.koeln) infrastructure should be merged [into this branch](https://git.pub.solar/pub-solar/os/src/branch/momo/main)
+- Changes can get accepted via: Pull Request
+- Deployment of changes is [automatic via CI pipeline](https://git.pub.solar/pub-solar/os/src/commit/43bd7421509f7cc9ba06d7c740f3f536a4a2af76/.drone.yml#L20-L38)
+- Branch protected from direct `git push`
+
+### `$USER` branches
+- User's custom hosts and changes can be worked on in these branches
+- Direct `git push` possible
+- Examples:
+    - [hensoko](https://git.pub.solar/pub-solar/os/src/branch/hensoko)
+    - [b12f](https://git.pub.solar/pub-solar/os/src/branch/b12f)
+    - [axeman](https://git.pub.solar/pub-solar/os/src/branch/axeman)
+    - [teutat3s](https://git.pub.solar/pub-solar/os/src/branch/teutat3s)