Add Contributing.md

Author: Ffloriel

CONTRIBUTING.md

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+## [Code of Conduct](./.github/CODE_OF_CONDUCT.md)
+
+FullHuman has adopted the Contributor Covenant Code of Conduct for all of its project.
+Please read the text so that you understand how to conduct while contributing to this project.
+
+## Semantic Versioning
+
+Purgecss use [SemVer](http://semver.org/) for versioning.
+
+## Sending a Pull Request
+
+**Before submitting a pull request,** please make sure the following is done:
+
+1. Fork [the repository](https://github.com/FullHuman/purgecss-webpack-plugin) and create your branch from `master`.
+2. If you've added code that should be tested, add tests!
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes (`npm test`).
+5. Make sure your code lints (`npm run lint`).
+
+
+### Development Workflow
+
+After cloning Purgecss, run `npm i` to fetch its dependencies.
+Then, you can run several commands:
+
+- `npm run dev` will build cjs and es module of Purgecss in the `lib` folder and watch for changes.
+- `npm run lint` checks the code style.
+- `npm test` runs the complete test suite.
+- `npm test -- --watch` runs an interactive test watcher.
+- `npm test <pattern>` runs tests with matching filenames.
+- `npm run build` creates the cjs and es module of Purgecss in the `lib` folder.
+
+Make sure that your pull request contains unit tests for any new functionality. This way we can ensure that we don't break your code in the future.
+
+### License
+
+This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.
+
+### Aknowledgement
+
+This CONTRIBUTING.md is based on the [react](https://github.com/facebook/react) [how-to-contribute.md](https://github.com/facebook/react/blob/master/docs/contributing/how-to-contribute.md) file.