Provide an example for detecting private messages

[esigler]

Here's a quick example for how to detect a private message to Lita - I think it's kind of overly verbose though. Is it worth having a "one liner" section for these kinds of things?

[jimmycuadra]

That is a lot of code just to demonstrate effectively one method call. I was thinking about adding a third section to the docs for "advanced plugin development" or "plugin development patterns" or something along those lines that could be sort of a grab bag examples on how to achieve certain tasks when writing plugins. That would also be a good place to address storing metadata in User objects like you brought up in litaio/lita#63.

[esigler]

Agreed - there's lots of little things I try to remember from plugin to plugin, will close this PR and open a new one for a "plugin development patterns" section.