adding CONTRIBUTING to backbone

CONTRIBUTING.md

@@ -0,0 +1,9 @@
+## How to contribute to Backbone.js
+
+* Before you open a ticket or send a pull request, [search](https://github.com/documentcloud/backbone/issues) for previous discussions about the same feature or issue. Add to the earlier ticket if you find one.
+
+* Before sending a pull request for a feature, be sure to have [tests](http://backbonejs.org/test/test.html).
+
+* Use the same coding style as the rest of the [codebase](https://github.com/documentcloud/backbone/blob/master/backbone.js).
+
+* In your pull request, do not add documentation or re-build the minified `backbone-min.js` file. We'll do those things before cutting a new release.