add a CONTRIBUTING.md file for github

CONTRIBUTING.md

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+## Contributing To Crossfilter
+
+### Individual Contributor License Agreement
+
+Want to add support for a new backend or visualization? We'd love for you to participate in the development of Crossfilter. Before we can accept your pull request, please sign our [Individual Contributor License Agreement][1]. It's a short form that covers our bases and makes sure you're eligible to contribute. Thank you!
+
+  [1]: https://spreadsheets.google.com/spreadsheet/viewform?formkey=dDViT2xzUHAwRkI3X3k5Z0lQM091OGc6MQ&ndplr=1
+
+### Getting Help
+
+Full API documentation is kept [in the wiki][2]. If you have a question or need help using Crossfilter, please ask on Stackoverflow using the [crossfilter tag][3]. Feel free to file bug reports and other suggestions in the [github issue tracker][4]. 
+
+  [2]: https://github.com/square/crossfilter/wiki
+  [3]: http://stackoverflow.com/questions/tagged/crossfilter
+  [4]: https://github.com/square/crossfilter/issues
+
+### License
+
+Crossfilter is available under the [Apache License][5].
+
+  [5]: https://github.com/square/crossfilter/blob/master/LICENSE
+
+