Add docs / guide for debugger

[cldwalker]

Motivated by #39, it would be good to have a guide that lives in doc/ that reflects debugger features that currently work. The doc should be in markdown and if desired could just be a rehash of what's on http://bashdb.sourceforge.net/ruby-debug.html and http://bashdb.sourceforge.net/ruby-debug/rdebug-emacs.html . Any help welcome :)

[science]

I'd also suggest adding some very basic start up docs to the main readme:

Getting Started

After installing the debugger gem, you can create a test file (in this example called "test-debug.rb") to try out the debugger functionality.

require 'debugger'

x = 4
debugger
y = 5

Running the above code as file in Ruby (e.g., ruby test-debug.rb) will cause debugger to break code execution and give you an interactive console on the line "debugger". You can "step" to follow the call or "next" to stay at the same level and process the line you're on. You can also make any IRB style commands you want to inspect or modify the code. Typing "continue" will resume code execution until the next debugger command is encountered, if any.