Add CODE_OF_CONDUCT file

This project's code of conduct is currently referenced from the README file. It's standard to have a
file in the root of the repo called CODE_OF_CONDUCT that includes this information.

GitHub understands this file and includes information about it in data returned by its API.

Author: drewnoakes

CODE_OF_CONDUCT.md

@@ -0,0 +1,5 @@
+# Code of Conduct
+
+This project has adopted the code of conduct defined by the Contributor Covenant
+to clarify expected behavior in our community.
+For more information, see the [.NET Foundation Code of Conduct](https://dotnetfoundation.org/code-of-conduct).