Add contributing file.

Author: Potherca

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing!
+
+All PDS Interop projects are open source and community-friendly.
+
+Please visit [https://pdsinterop.org/contributing/](https://pdsinterop.org/contributing/) for details about:
+
+- Our [Code of Conduct](https://pdsinterop.org/code-of-conduct/)
+- How to report issues
+- How to make code changes
+- How to send a merge request