include rb-fsevent in gemfile

if I don't have gem 'rb-fsevent', '~> 0.9' in my gemfile I get this warning:

Listen warning]:
  Missing dependency 'rb-fsevent' (version '~> 0.9')!
  Please add the following to your Gemfile to satisfy the dependency:
    gem 'rb-fsevent', '~> 0.9'

  For a better performance, it's recommended that you satisfy the missing dependency.
  Listen will be polling changes. Learn more at https://github.com/guard/listen#polling-fallback.


not sure why so I suggest it gets added in or at least documented