add mysql engine config example

It's the same as postgresql because of the used backend but an example
prevents some guessing.

etc/backy.cfg

@@ -43,6 +43,7 @@ type: backy2.meta_backends.sql
 #   sqlite:////path/to/sqlitefile
 #   postgresql:///database
 #   postgresql://user:password@host:port/database
+#   mysql://user:password@host:port/database
 engine: sqlite:////var/lib/backy2/backy.sqlite