Added some more config documentation

Dear bundler-site maintainer,

I added some documentation for the "bundle config" command. I would love to add some more, but I just wanted to check whether this documentation is OK, so I can expand it later.

Please be as critical as you'd like as I want to add a lot more.

Question:
Should the examples be more specific? E.g. should I get/set the "bin" variable for example?

Edit: Removed question on running the site. It runs well with <pre>rackup config.ru</pre>