started contributing instructions

Author: unitycoder

CONTRIBUTING.md

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+How Can I Contribute?
+
+- Report Bugs, Requests or Suggestions using https://github.com/UnityCommunity/UnityLibrary/issues
+- Adding Content to Wiki https://github.com/UnityCommunity/UnityLibrary/wiki
+- Updating, Optimizing or Arranging/Cleaning-up existing content
+- Submitting fixes or new content using Pull Request (or joining as a member and directly adding to this repository)