Create CONTRIBUTING.md (#6386)

This file exists in corefx and is an obvious file to look for. Making the two repos the same.

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
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+# Contributing
+
+See [Contributing](Documentation/project-docs/contributing.md) for information about coding styles, source structure, making pull requests, and more.
+
+# Developers
+
+See the [Developer Guide](Documentation/project-docs/developer-guide.md) for details about developing in this repo.