Print help message when user joins

[mgyucht]

From what I can tell, this might be kind of tough since the current outgoing webhooks just posts to your webserver when messages are sent in the channel. However, I remember seeing a blog post where they had an integration which responded to users joining a channel... worth trying to find it.

[mgyucht]

This is probably blocked on making the bot use the RTM API or being a bot user.