Explain how ScalatraBootstrap is loaded

[rossabaker]

The key to override it is explained at http://scalatra.org/2.2/guides/deployment/standalone.html. We should better explain its requirements:

• implements LifeCycle
• has public, no-arg constructor so it can be instantiated through reflection

[futurechimp]

Hiya Ross,

I'm working through a few docs tickets here. I suspect this one comes from a specific query on the list or in IRC? I am not sure what changes to make to improve the docs on this one, so I'll leave this one to you (or whoever else knows what it's all about).