Add contribution guide with release instructions (#7)

* Add contribution guide with release instructions

* Update wording from suggestion

Co-authored-by: Simon Gerber <simon.gerber@vshn.ch>

Co-authored-by: Simon Gerber <simon.gerber@vshn.ch>

CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# How to contribute
+
+This code repository is part of Project Syn and the contribution guide at
+https://syn.tools/syn/about/contribution_guide.html does apply.
+
+Submit Pull Requests at https://github.com/projectsyn/getting-started-commodore-defaults/pulls.
+
+## Releasing
+
+Tags in this repository should be used to specify the [Commodore version](https://github.com/projectsyn/commodore/releases) with which the configuration has been written/tested.
+The tags are used in the [Getting Started Guide](https://syn.tools/syn/tutorials/getting-started.html) to ensure a matching Commodore version and default configuration are used.
+
+When updating this repository, you should check if the  [Getting Started Guide](https://syn.tools/syn/tutorials/getting-started.html) needs to be updated as well.