Fix bundle configuration for first time users

I noticed that when starting anew with the repository, following the instructions in README.md generates an error when trying to run bundle install:

Your user account isn't allowed to install to the system RubyGems.
  You can cancel this installation and run:

      bundle config set --local path 'vendor/bundle'
      bundle install

  to install the gems into ./vendor/bundle/, or you can enter your password
  and install the bundled gems to RubyGems using sudo.

From the content of .gitignore, I assume that most of you use the .bundle/ directory. This MR adds a config file to .bundle/ that lets bundle know where to install the gems.