doc: add contribution info

Author: FooDeas

CONTRIBUTING.md

@@ -0,0 +1,26 @@
+# Contributing
+
+Bug fixes and improvements are always welcome!
+
+## Commit format
+
+Prefix your commits with one of these strings followed up by a whitespace:
+
+| Prefix | Example |
+|--------------|----------------------------------------------------------------------------------------|
+| `build:` | changes in build process |
+| `doc:` | changes in documentation files |
+| `installer:` | changes in installation process (can contain documentation changes if belonging to it) |
+| `git:` | changes in `.gitignore` |
+| `all:` | changes that have high impact on installer structure (moved directories, ...) |
+
+Example: `doc: fix typo in readme`
+
+## Pull requests
+
+Please do your pull requests always against the [devel branch](https://github.comhttps://github.com/FooDeas/raspberrypi-ua-netinst/tree/devel).
+
+**Note:**  
+The [devel branch](https://github.com/FooDeas/raspberrypi-ua-netinst/tree/devel) should always contain stable and tested versions!
+
+If the [devel branch](https://github.com/FooDeas/raspberrypi-ua-netinst/tree/devel) has "enough" or important changes, it will (selectively) get merged into [master](https://github.com/FooDeas/raspberrypi-ua-netinst), [tagged](https://github.com/FooDeas/raspberrypi-ua-netinst/tags) and [released](https://github.com/FooDeas/raspberrypi-ua-netinst/releases).