Use bundle add instead

[glaucocustodio]

As per rubygems/rubygems#5337, we can simplify the steps of adding a gem.

[searls]

Thanks for the suggestion, but adding the Gemfile stanza is pretty standard for READMEs and I have never seen people use bundle add (nor would I use it, since I'm pretty precious about how I format my Gemfiles). It would preclude folks using alternate tools like gel