Added detail to installation for ruby versions.

Based on issue #23 and issue #12, it seems some people aren't too clear on how this works with various ruby versions. I tried to make it more clear in the installation section.

I am not sure how to get your plugin working with unsupported ruby versions (couldn't get it to work with 2.2.0 since it seems the byebug gem does not load) so while I did add how to get past the version check I did not write more. I think you need to also change ```/sublime_debug_require.rb``` and ```/debugger/ruby_imp/ruby_debugger.py``` to support new versions. 

Right now, it seems you hardcoded the ruby versions. I would suggest you consider putting a flag somewhere (maybe in ```ruby_version_discoverer.rb```) where the user can change it so the plugin dynamically chooses if to use ruby debug vs byebug for a range of versions.