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.

Questions:
1) This is embarrassing to ask, but how can I run the site so I can see my changes?
2) Should the examples be more specific? E.g. should I get/set the "bin" variable for example?