Add contributing guidelines

Regulate git commit style and ban LLM usage.

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing guidelines for jungle
+
+We welcome contributions from other people, provided that they follow these
+guidelines.
+
+## Git commit rules
+
+The commit title must be strictly 50 character or less, the commit body must
+wrap at 72 characters.
+
+Every commit to be merged in master must be reviewed by at least one other
+member and have the corresponding `Reviewed-by: John Doe <jd@ema.il>` git
+trailer with their name and email.
+
+Commits that fix a particular issue must include the **full URL** to the issue
+in the `Fixes: <url>` trailer.
+
+## LLM are banned
+
+Contributions generated fully or partially by LLMs won't be accepted, you must
+write the code yourself and fully understand what every part does.
+
+Using LLMs to help preparing a contribution is strongly discouraged, as it will
+likely cause you to have gaps of understanding.

README.md

@@ -7,3 +7,5 @@ This repository provides two components that can be used independently:
 
 - NixOS configurations for jungle machines. Use `nixos-rebuild switch --flake .`
   to upgrade the current machine.
+
+Before contributing, please read the [guidelines](CONTRIBUTING.md).