added contributor agreement

Author: Mark Visser

CONTRIBUTING.md

@@ -0,0 +1,9 @@
+# Contributing
+
+## If you are interested in contributing, here are some ground rules:
+* ... Define guidelines & rules for what contributors need to know to successfully make Pull requests against your repo ...
+
+## All contributions are subject to the [Unity Contribution Agreement(UCA)](https://unity3d.com/legal/licenses/Unity_Contribution_Agreement)
+By making a pull request, you are confirming agreement to the terms and conditions of the UCA, including that your Contributions are your original creation and that you have complete right and authority to make your Contributions.
+
+## Once you have a change ready following these ground rules. Simply make a pull request