adding contribution guidelines

CONTRIBUTING.md

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+# How to Contribute
+
+We'd love to get patches from you!
+
+## Getting Started
+
+We follow the [GitHub Flow Workflow](https://guides.github.com/introduction/flow/)
+
+1.  Fork the project
+1.  Check out the `master` branch
+1.  Create a feature branch
+1.  Write code and tests for your change
+1.  From your branch, make a pull request against `twitterdev/web-recipes/master`
+1.  Work with repo maintainers to get your change reviewed
+1.  Wait for your change to be pulled into `twitterdev/web-recipes/master`
+1.  Delete your feature branch
+
+# License
+
+By contributing your code, you agree to license your contribution under the
+terms of the APLv2: https://github.com/twitterdev/web-recipes/blob/master/LICENSE
+
+# Code of Conduct
+
+Read our [Code of Conduct](CODE_OF_CONDUCT.md) for the project.