docs: Add contributing docs (#173)

CONTRIBUTING.md

@@ -1,5 +1,38 @@
 # Contributing
 
+## Installing Dependencies
+
+We use [pnpm](https://pnpm.io) for managing dependencies. You can install the
+necessary dependencies using the following command:
+
+```bash
+pnpm install
+```
+
+## Running Tests
+
+When making changes to lint rules, you can re-run the tests with the following
+command:
+
+```bash
+pnpm test
+```
+
+Or run it in watch mode like so:
+
+```bash
+pnpm test -- --watch
+```
+
+## Adding new rules
+
+When adding new rules, make sure to follow these steps:
+
+1. Add the rule source code in `src/rules`
+1. Add tests for the rule in `test/spec`
+1. Add docs for the rule to `docs/rules`
+1. Add a short description of the rule in `README.md`
+
 ## Releasing
 
 To release a new version with semantic-release, run the following command.