Make `help` command more useful

[jdm]

It currently responds with a link to the Servo documentation, but I've never seen the command used for that purpose. Users are more likely to want information about crowbot's features, so we should provide that (probably by linking to a document in this repository).

[cs40]

Hi @jdm ,
I would like to work on this issue. How would I get started?

[jdm]

@cs40 You can find the code that responds to commands at https://github.com/servo/crowbot/blob/master/server.js#L180 . You'll want to run your own instance of the bot to experiment; I think the best choice would be to reply with a link to https://github.com/servo/crowbot/wiki/Commands, which will contain the most up to date documentation.