Add CONTRIBUTING.md file.

Author: pazguille

CONTRIBUTING.md

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+# Contributing
+
+## Code style
+Regarding code style like indentation and whitespace, **follow the conventions you see used in the source already.**
+
+## Modifying the code
+
+Please don't edit `/dist/slideout.js` and `/dist/slideout.min.js` files. You'll find source code in the `index.js` file.
+
+1. Fork and clone the repo.
+2. Run `npm install` to install all dependencies.
+3. Create a new branch, please don't work in your `master` branch directly.
+4. Open the file `/test/index.html`.
+5. Code!
+
+### Running the tests
+
+- Run `npm test` from your command line and check the console
+- Open the `/test/index.html` file to run test in your browser.
+
+## Pull requests
+
+1. Create a new branch, **please don't work in your `master` branch directly**.
+2. Code!
+3. Update the tests and run `npm test` to see the tests.
+4. Run `npm run hint`.
+5. Run `npm run dist` to build a new version.
+6. Update the documentation and package to reflect any changes.
+7. Push to your fork and submit a pull request.