Update CHANGES.md for #3516

Author: echoix

CHANGES.md

@@ -65,6 +65,8 @@
 
 <!-- Changes to Black's terminal output and error messages -->
 
+- Calling `black --help` multiple times will return the same help contents each
+  time (#3516)
 - Verbose logging now shows the values of `pyproject.toml` configuration variables
   (#3392)
 - Fix false symlink detection messages in verbose output due to using an incorrect