Adding note on command line tool

CHANGELOG.md

@@ -37,6 +37,8 @@ Added new web-based System Console for managing instance level configuration. Th
 
 In addition new command line tools are available for managing Mattermost system roles, creating users, resetting passwords, getting version info and other basic tasks. 
 
+Run `./platform -h` for documentation using the new command line tool.
+
 
 ### New Features