Created CONTRIBUTING.md. Credit to @lmh for the initial text.

CONTRIBUTING.md

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+## Filing an issue
+
+Before filing an issue, please [search the queue](https://github.com/masayuki0812/c3/issues) to make sure it hasn't already been reported.
+
+If a bug, please include the following —
+
+1. What version of C3?
+1. What browsers have you confirmed it in?
+1. Can you isolate the issue by providing a jsFiddle demonstrating it in a minimalist capacity?
+
+Please *do not* ask for support using the issue queue. For support, please ask [on chat](https://gitter.im/masayuki0812/c3) or [the mailing list](groups.google.com/forum/#!forum/c3js).
+
+##Building C3 from sources
+
+ 1. **Clone the repo from GitHub**
+
+        git clone https://github.com/masayuki0812/c3.git
+        cd c3
+
+ 2. **Acquire build dependencies.** Make sure you have [Node.js](http://nodejs.org/) installed on your workstation. This is only needed to _build_ C3 from sources. C3 itself has no dependency on Node.js once it is built. Now run:
+
+        npm install -g grunt-cli
+        npm install
+
+    The first `npm` command sets up the popular [Grunt](http://gruntjs.com/) build tool. You might need to run this command with `sudo` if you're on Linux or Mac OS X, or in an Administrator command prompt on Windows. The second `npm` command fetches the remaining build dependencies.
+
+ 3. **Run the build tool**
+
+        grunt
+
+    Now you'll find the built files in `c3.js`, `c3.min.js`, `c3.css` & `c3.min.css`.
+
+## Running the tests
+The `grunt` script will automatically run the specification suite and report its results.
+
+Or, if you want to run the specs in a browser (e.g., for debugging), simply open `spec/runner.html` in your browser.
+
+## Contributing your changes
+
+Add something about PRs here, indicate that PRs should not bump the version number & the build output files (`c3.js`, `c3.min.js`, `c3.css` & `c3.min.css`) should be excluded