chore: move Contribution guide to project root

CONTRIBUTING.md

@@ -0,0 +1,72 @@
+# Contribution Guide
+
+## Setting up the project
+
+Use [nvm](https://nvm.sh) to use the appropriate Node.js version from `.nvmrc`:
+```sh
+nvm i
+```
+
+Install the dependencies using pnpm:
+```sh
+pnpm i
+```
+
+## Building the project
+Run the `build` script:
+```sh
+pnpm build
+```
+
+The package is bundled using [pkgroll](https://github.com/privatenumber/pkgroll) (Rollup). It infers the entry-points from `package.json` so there are no build configurations.
+
+
+### Development (watch) mode
+During development, you can use the watch flag (`--watch, -w`) to automatically rebuild the package on file changes:
+```sh
+pnpm build -w
+```
+
+## Running the package locally
+Since pkgroll knows the entry-point is a binary (being in `package.json#bin`), it automatically adds the Node.js hashbang to the top of the file, and chmods it so it's executable.
+
+You can run the distribution file in any directory:
+```sh
+./dist/cli.mjs
+```
+
+Or in non-UNIX environments, you can use Node.js to run the file:
+```sh
+node ./dist/cli.mjs
+```
+
+## Testing
+
+Testing requires passing in `OPENAI_KEY` as an environment variable:
+
+```sh
+OPENAI_KEY=<your OPENAI key> pnpm test
+```
+
+
+You can still run tests that don't require `OPENAI_KEY` but will not test the main functionality:
+```
+pnpm test
+```
+
+
+## Using & testing your changes
+You can publish the package to a GitHub branch using [`git-publish`](https://github.com/privatenumber/git-publish):
+
+Publish your current branch to a GitHub branch:
+```sh
+$ npx git-publish
+
+✔ Successfully published branch! Install with command:
+  → npm i 'Nutlope/aicommits#npm/develop'
+```
+
+Then you can use the output to test the changes:
+```sh
+$ npx 'Nutlope/aicommits#npm/develop' # same as running `npx aicommits`
+```