docs: Add contribution guide (#76)

CONTRIBUTING.md

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+# Contributing
+
+Everyone is welcome to contribute to WXT!
+
+If you are changing the docs or fixing a bug, feel free to fork and open a PR.
+
+If you want to add a new feature, please create an issue or discussion first so we can decide if the feature is inline with the vision for WXT.
+
+## Conventional Commits
+
+This project uses [Conventional Commits](https://www.conventionalcommits.org/en) to automate versioning. If you're a new contributor, don't worry about this. When you open a PR, a maintainer will change the PR's title so it's in the style of conventional commits, but that's all.
+
+Maintainers, commits to the `main` branch (either directly or via PRs) must be valid conventional commits.
+
+## Setup
+
+WXT uses `pnpm`, so make sure you have it installed.
+
+```sh
+corepack enable
+```
+
+Then, simply run the install command:
+
+```sh
+pnpm i
+```
+
+## Development
+
+Here are some helpful commands:
+
+```sh
+# Build WXT package
+pnpm build
+```
+
+```sh
+# Build WXT package, then build demo extension
+cd demo
+pnpm build
+```
+
+```sh
+# Build WXT package, then start the demo extension in dev mode
+cd demo
+pnpm dev
+```
+
+```sh
+# Run unit and E2E tests
+pnpm test
+```
+
+```sh
+# Start the docs website locally
+pnpm docs:dev
+```
+
+## Testing
+
+WXT has unit and E2E tests. When making a change or adding a feature, make sure to update the tests or add new ones.
+
+To run tests for a specific file, add the filename at the end of the test command:
+
+```sh
+pnpm test manifest-contents
+```
+
+Unit and E2E tests are ran together via [Vitest workspaces](https://vitest.dev/guide/#workspaces-support).
+
+If you want to manually test a change, you can modify the demo project for your test, but please don't leave those changes committed once you open a PR.