[backport] add CODE_OF_CONDUCT.md

Author: SethTisue

CODE_OF_CONDUCT.md

@@ -0,0 +1,7 @@
+all repositories in these organizations:
+
+* [scala](https://github.com/scala)
+* [scalacenter](https://github.com/scalacenter)
+* [lampepfl](https://github.com/lampepfl)
+
+are covered by the Scala Code of Conduct: https://scala-lang.org/conduct/