[docs] add CONTRIBUTING.md (#58)

CONTRIBUTING.md

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+As an open source project we welcome contributions of many forms, but due to the
+experimental pre-beta nature of this repository, you should reach out before starting
+work on any major code changes. This will ensure we avoid duplicating work, or that
+your code can't be merged due to a rapidly changing base. If you would like support
+for a module that is not listed, [contact support][1] to share a request.
+
+[1]: https://docs.datadoghq.com/help