Use `bundle add` in README installation instructions (#1287)
Use `bundle add` in README installation instructions (#1287)

Use `bundle add` in README installation instructions, to reduce the steps needed to start using the gem.

Co-authored-by: David Rodríguez <96558bf9378c437d3852fad4fa4b4bd63aeb6eda@riseup.net>
activerecord-hackery/ransack