Add contributor documentation based on webpack's one

In order to allow others to contribute to the
project, we need to specify the rules and
workflows with which we work. Adding them now in
the right place.

CONTRIBUTING.md

@@ -0,0 +1,67 @@
+# Contributing
+
+From opening a bug report to creating a pull request: every contribution is
+appreciated and welcomed. If you're planning a new feature or change
+the api please create an issue first. This way we can ensure that your precious
+work is not in vain.
+
+## Issues
+
+Most of the time, when webpack does not work correctly, it might be a configuration issue.
+
+If you are still having difficulty after looking over your configuration carefully, please post
+a question to [StackOverflow with the webpack-cli tag](http://stackoverflow.com/tags/webpack-cli). Questions
+that include your webpack.config.js and relevant files, this way you help others to help you.
+
+**If you have discovered a bug or have a feature suggestion, feel free to create an issue on Github.**
+
+## Setup
+
+* Install [Node.js](https://nodejs.org/) if you don't have it already.
+  *Note: Node 6 or greater would be better for "best results".*
+* Fork the **webpack-cli** repo at [https://github.com/webpack/webpack-cli](https://github.com/webpack/webpack-cli).
+* `git clone <your-clone-url> && cd webpack-cli`
+* `git checkout develop`
+
+### Setup with npm
+* Install the dependencies: `npm install`
+* Run the tests with: `npm test`
+
+### Setup with yarn
+* If you don't have yarn yet: `npm install -g yarn`
+* Install the dependencies and link them
+
+```bash
+yarn install
+yarn link
+yarn link webpack-cli
+```
+
+To run the entire test suite use:
+
+```bash
+yarn test
+```
+
+## Editor Config
+
+The [.editorconfig](https://github.com/webpack/webpack-cli/blob/master/.editorconfig) in the root should ensure consistent formatting. Please make sure you've [installed the plugin](http://editorconfig.org/#download) if your text editor needs one.
+
+## Submitting Changes
+
+After getting some feedback, push to your fork and submit a pull request. We
+may suggest some changes, improvements or implementation alternatives.
+
+In case you've got a small change in most of the cases your pull request would be accepted quicker.
+
+### How to increase chance of having a pull request accepted?
+
+* Write tests
+* Follow the existing coding style
+* Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
+
+## Documentation
+
+webpack is insanely feature rich and documentation is a time sink. We
+greatly appreciate any time spent fixing typos or clarifying sections in the
+documentation.