Add a code of conduct document

This just refers to the CNCF code of conduct instead of containing
the details directly, so that we don't need to change anything if
the CNCF document changes.

Signed-off-by: Jon Skeet <[email protected]>

Author: jskeet

CODE_OF_CONDUCT.md

@@ -0,0 +1,6 @@
+# Code of Conduct
+
+The CloudEvents C# SDK is committed to the [CNCF Code of
+Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).
+Please refer to that document for details of the standards, reporting processes and
+enforcement.