This page will determines what Yuki should send in where.
So, the logs will not gonna be sent to that channel until the logEvents doesn't have a true value. So, you can edit the module by typing
y!config edit logEvents <true/false>
By enabling the events (which is required) now, you need to provide a channel to complete the event. You can edit the module by typing
y!config edit LogChannel <yourchannelname/yourchannelid>
And there you go! If a user leaves/join, Yuki will sends a notification about it into the selected channel.