Adds missing CONTRIBUTING.md file

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing
+
+Contributions are **welcome** and will be fully **credited**.
+
+We accept contributions via Pull Requests on [Github](https://github.com/spatie/laravel-mix-purgecss).
+
+## Pull Requests
+
+- Use the ES6 syntax.
+- Your patch won't be accepted if it doesn't pass the tests and lints (`npm run test`).
+- If there's a `/demo` section, try to add an example.
+- **Document any change in behaviour:** Make sure the `README.md`, `CHANGELOG.md` and any other relevant documentation are kept up-to-date.
+- **Consider our release cycle:** We try to follow [SemVer v2.0.0](http:https://semver.org/). Randomly breaking public APIs is not an option.
+- **Create feature branches:** Don't ask us to pull from your master branch.
+- **One pull request per feature:** If you want to do more than one thing, send multiple pull requests.
+- **Send coherent history:** Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash them](http:https://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
+
+## Running Tests
+
+```bash
+jest
+```
+
+**Happy coding**!