Add contributing guidelines

CONTRIBUTING.md

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+# Contributing to Fabric
+
+1. [Questions?!?](#questions)
+2. [Issue tracker](#issue-tracker)
+3. [Issue tracker guidelines](#issue-tracker-guidelines)
+4. [Pull requests](#pull-request)
+5. [Pull request guidelines](#pull-request-guidelines)
+
+## Questions?!?
+
+To get your questions answered, please ask/search on [Fabric's google group], [stackoverflow] or on Fabric's IRC channel (irc://irc.freenode.net/#fabric.js).
+Please do not open an issue if you're not sure it's a bug or a suggestion.
+
+For news about Fabric you can follow [@fabric.js] or [@kangax] on twitter.
+Demos and examples can be found on [jsfiddle], [codepen.io] and [fabricjs.com].
+
+## Issue tracker
+
+If you are sure that it's a bug in Fabric.js or a suggestion, open a new [issue] and try to
+answer the following questions:
+
+- What did you do?
+- What did you expect to happen?
+- What happened instead?
+
+### Issue tracker guidelines
+
+- **Search:** Before opening a new issue please [search](https://github.com/kangax/fabric.js/search?q=&ref=cmdform&type=Issues) Fabric's issues.
+
+- **Title:** Choose a informative title.
+
+- **Description:** Use the questions above to describe the issue. Add logs, screenshots or videos if that makes sense.
+
+- **Test case:** Please post code to replicate the bug - best on [jsfiddle](http://jsfiddle.net). Ideally a failing test would be
+perfect, but even a simple script demonstrating the error would suffice.
+You could use [this jsfiddle template](http://jsfiddle.net/fabricjs/Da7SP/) as a
+starting point.
+
+- **Fabric.js version:** Make sure to specify which version of Fabric.js you are using. The version can be found in [all.js file](https://github.com/kangax/fabric.js/blob/master/dist/all.js#L5) or
+just enter `fabric.version` in the browser console.
+
+
+## Pull requests
+
+We are very grateful for pull requests from you! You have the chance to improve Fabric.
+
+### Pull request guidelines
+
+Here are a few notes you should take into account:
+
+- **Code style, notes:** Make sure that you have complied with the [guidelines](https://github.com/kangax/fabric.js/wiki/How-to-contribute-to-Fabric#code-style-notes)
+
+- **Distribution files:** Do your changes only in the [source files](https://github.com/kangax/fabric.js/tree/master/src). Don't include the [distribution files](https://github.com/kangax/fabric.js/tree/master/dist) in your commit.
+
+- **Add tests**: Tests would be awesome - invest a little time and expand the [unit tests](https://github.com/kangax/fabric.js/tree/master/test/unit). More informations about qunit tests can be found in the [wiki](https://github.com/kangax/fabric.js/wiki/How-to-contribute-to-Fabric#testing-fabric).
+
+- **Add documentation:** Fabric uses [JSDoc 3] for documentation. The generated documentation can be found [fabricjs.com](http://fabricjs.com/docs).
+
+- **Create topic branches.** Don't use your master branch for pull request. It's better to create a new branch for every pull request.
+
+- **One pull request per feature/bug**. If you want to do more than one thing, send
+  multiple pull requests.
+
+- **And there you go!** If you still have questions we very glad to help you or have a look at the [wiki](https://github.com/kangax/fabric.js/wiki/How-to-contribute-to-Fabric).
+
+[Fabric's google group]: https://groups.google.com/forum/#!forum/fabricjs
+[stackoverflow]: http://stackoverflow.com/questions/tagged/fabricjs
+[@fabric.js]: https://twitter.com/fabricjs
+[@kangax]: https://twitter.com/kangax
+[jsfiddle]: http://jsfiddle.net/user/fabricjs/fiddles
+[codepen.io]: http://codepen.io/tag/fabricjs
+[fabricjs.com]: http://fabricjs.com/demos
+[JSDoc 3]: http://usejsdoc.org/
+[issue]: https://github.com/kangax/fabric.js/issues