Add OSS management files. (#130)

Inspired by Docker and Yarn. We can add more things later if we need
them, but I think this is a good start.

Signed-off-by: David Calavera <david.calavera@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# CONTRIBUTING
+
+Contributions are always welcome, no matter how large or small. Before contributing,
+please read the [code of conduct](CODE_OF_CONDUCT.md).
+
+## Setup
+
+> Install yarn on your system: https://yarnpkg.com/en/docs/install
+
+```sh
+$ git clone https://github.com/netlify/netlify-cms
+$ cd netlify-cms
+$ yarn
+```
+
+## Building
+
+```sh
+$ npm run build
+```
+
+## Testing
+
+```sh
+$ npm run test
+```
+
+```sh
+$ npm run test:watch
+```
+
+```sh
+$ npm run lint
+```
+
+## Runing the server
+
+```sh
+$ npm run start
+```
+
+## Pull Requests
+
+We actively welcome your pull requests.
+
+1. Fork the repo 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.
+5. Make sure your code lints.
+
+## License
+
+By contributing to Netlify CMS, you agree that your contributions will be licensed
+under its [MIT license](LICENSE).