Add CODE-OF-CONDUCT.md

Fixes #18820

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19002)

CODE-OF-CONDUCT.md

@@ -0,0 +1,6 @@
+Code of Conduct
+===============
+
+The OpenSSL [Code of Conduct] is published on the project's website.
+
+[Code of Conduct]: https://www.openssl.org/community/conduct.html