Include Code of Conduct in GitHub-recognised manner. (dotnet#25751)

This is just a link to the code of conduct at
http://www.dotnetfoundation.org/code-of-conduct which is already linked
to in the readme.

Putting it at this location just has github recognise it as such
(https://github.com/dotnet/corefx/community)

The text is the same as that used at
https://github.com/dotnet/docs/blob/master/CODE_OF_CONDUCT.md

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).