docs: add CONTRIBUTING

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing guide
+
+Things you'll need:
+
+- To create [Pull Requests](https://forgejo.org/docs/latest/user/pull-requests-and-git-flow/), you will need to register your [pub.solar ID](https://auth.pub.solar) first.
+- For small changes, e.g. to documentation, you can directly edit files in the browser.
+
+### Getting a development shell
+
+First, get a local copy of this repository:
+
+```
+git clone https://git.pub.solar/pub-solar/infra.git
+cd infra
+```
+
+then, install [the package manager nix](https://nixos.org/download).
+
+Finally, run:
+
+```
+nix develop
+```
+
+This will install a development shell (devshell) with all required tools.
+
+### Final checks before creating a Pull Request
+
+Before creating a pull request, it's recommended to check the formatting:
+
+```
+treefmt
+```
+
+If you are a terminal-lover, the [AGit alternative](https://forgejo.org/docs/latest/user/agit-support/) to the web based Pull Request workflow might be interesting.