doc: specify commit message format

I've noticed quite a few pull requests are being submitted with formats
such as feat(abc): or target: at the start of the message.

It will be helpful to have a specific format for this to keep things
consistent.

CONTRIBUTING.md

@@ -0,0 +1,36 @@
+# Contributing to Stylix
+
+## Commit messages
+
+To keep things consistent, commit messages should follow a format
+[similar to Nixpkgs](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#commit-conventions):
+
+```
+«scope»: «summary»
+
+«motivation for change»
+```
+
+Where the scope is one of:
+
+| Scope          | Purpose                                                                |
+|----------------|------------------------------------------------------------------------|
+| `doc`          | Changes to the website, `README.md`, and so on.                        |
+| `stylix`       | Changes in the `stylix` directory, `flake.nix`, and other global code. |
+| Name of target | Changes to code for a particular target.                               |
+| `treewide`     | Changes across many targets.                                           |
+
+The scope is meant to indicate which area of the code was changed. Specifying
+the type of change, such as `feat` or `fix`, is not necessary. Dependency
+updates should use whichever scope they are related to.
+
+The summary should start with a lowercase letter, and should not end with
+punctuation.
+
+Most commits to `master` will also include a pull request number in brackets
+after the summary. GitHub adds this automatically when creating a squash merge.
+
+### Old commits
+
+Commits before 2024 did not follow any particular format.
+Some have emojis from [GitMoji](https://gitmoji.dev).

docs/default.nix

@@ -43,6 +43,7 @@ in pkgs.stdenvNoCC.mkDerivation {
 
   patchPhase = ''
     cp ${../README.md} src/README.md
+    cp ${../CONTRIBUTING.md} src/contributing.md
 
     # The "declared by" links point to a file which only exists when the docs
     # are built locally. This removes the links.

docs/src/SUMMARY.md

@@ -13,5 +13,6 @@
 
 # Contributing
 
+- [Contributing](contributing.md)
 - [Adding modules](modules.md)
 - [Style guide](styling.md)