Add contributing file.

CONTRIBUTING.md

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+Contributing
+============
+
+If you would like to contribute code to Flow you can do so through GitHub by
+forking the repository and sending a pull request.
+
+When submitting code, please make every effort to follow existing conventions
+and style in order to keep the code as readable as possible. Please also make
+sure your code compiles by running `mvn clean verify`.
+
+Before your code can be accepted into the project you must also sign the
+[Individual Contributor License Agreement (CLA)][1].
+
+
+ [1]: https://spreadsheets.google.com/spreadsheet/viewform?formkey=dDViT2xzUHAwRkI3X3k5Z0lQM091OGc6MQ&ndplr=1