Adding CONTRIBUTING.md which clearly lays out that we are not accepting contributions.

CONTRIBUTING.md

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+# Contributing
+
+At this time, the repo is not accepting external contributions in the form of PRs.The Perception package and all of its features are in active development, and significant changes are expected in future releases.
+
+We encourage you to submit bugs, suggestions, and feedback as [GitHub issues](https://github.com/Unity-Technologies/com.unity.perception/issues).