Couple of suggestions for further docs

[jonlives]

Couple of suggestions for improvements to make to the documentation:

• List the supported formatter methods and what exactly they do - they're only referred to in the screenshot of the email, and some like additional_info and additional_details have very similar names.
• Document an example of the workflow involved in taking an alert, figuring out what fields it has, adding a new formatter, and a helper that formatter uses.

The documentation is pretty comprehensive, but if you don't already know nagios-herald it's a bit hard to figure out exactly what you're trying to do.

[RyanFrantz]

Had another convo with Cowie about docs. He had an issue when writing a helper where the global config was not available. That's because Config.load only gets called in Executor, by the nagios-herald. The docs need to make this clear.

Also, helpers.rb needs to be updated if a new helper is written. This could even be streamlined by loading all helpers in the helpers/ directory.