Add Contribution instructions (#91)

Author: ivargrimstad

CONTRIBUTING.md

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+# Source Code Submissions
+We welcome your contributions and look forward to collaborating with you. We can only accept source code repository
+submissions from users who have signed the [MVC 1.0 Contributor License Agreement](https://gist.github.com/ivargrimstad/e82172b9078040e721ce3628651b7df2).
+
+You will will be automatically instructed to sign the CLA when submitting your first Pull Request.
+
+**Note that while we are working out the details, we will only accept Pull Requests from Expert Group Members**.
+
+In the meantime we welcome you to contribute to Ozark and/or the TCK.