Add debugging instructions

[iox]

Currently fixing a bug in Hobo can only be done by very few "initiated" people, leaving a lot of motivated developers unsatisfied. We should make this process super easy, creating a small tutorial about fixing a bug like https://github.com/Hobo/hobo/issues/54 in a fun and interesting way:

• Using better errors
• Navigating through the Hobo source
• Playing around in the Hobo source
• "Connecting" the Hobo source to our application (using ":path" in Gemfile)
• Fixing the bug
• Running the integration tests
• Sharing the solution

[iox]

A first try: http://youtu.be/6aVGdyR0-E0