docs: begin contributing guidelines

Add guidelines for adding ports and Conventional Commits to help out new contributors.

CONTRIBUTING.md

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+# Contributing
+
+## Adding a port
+
+Create a file in `modules/<module>/` with the name of the port. Add the file to the
+`imports` declaration in `modules/<module>/default.nix`. All ports should have the
+`catppuccin.enable` and `catppuccin.flavour` options, and optionally the
+`catppuccin.accent` option. `catppuccin.flavour` and `catppuccin.accent` should
+default to `config.catppuccin.flavour` and `config.catppuccin.accent`, respectively.
+
+## Commit messages
+
+This repository uses [Conventional Commits](https://conventionalcommits.org).
+All commits should include a body that briefly describes the motivation and content
+of the commit.
+
+### Commit types
+
+- `fix`: A bug fix that doesn't modify the public API
+- `feat`: A code change that modifies the public API
+- `refactor`: A code change that doesn't change behavior
+- `style`: A style fix or change
+- `docs`: Any change to documentation
+- `ci`: Any change to CI files
+- `revert`: A revert commit. The message should describe the reasoning and the
+  commit should include the `Refs:` header with the short hashes of the commits
+  being reverted.
+- `chore`: catch-all type
+
+### Commit scopes
+
+For the commit scope, put the name of the port the commit affects. If the commit
+does not affect a specific port, omit the scope.
+
+### Breaking changes
+
+All breaking changes should be documented in the commit footer in the format
+described by Conventional Commits. Use the `<type>!` syntax in order to distinguish
+breaking commits in the log, but include the footer to provide a better description
+for the changelog generator.
+
+```
+feat(bar)!: foo the bars
+
+BREAKING CHANGE: bars are now foo'ed
+```