Document that commands use the RTM API as opposed to custom Slash Commands

[sedders123]

For new users of phial with experience of Slack, a "command" is normally preceded by a slash and they may expect that phial "command"s create slash commands via Slacks API. This is not the case as phial "command"s are executed using normal slack messages (often with a prefix), more like an IRC bot.

We should document this difference clearly to help reduce potential confusion.