Polling fallback message help

Motivated by: #284. [ci skip]
The full steps here would be (with Sass as an example):
1. create a Gemfile with the line:
```ruby
gem 'listen', '~> 2.8'
```
2. run `bundle update`
3. run `bundle exec sass --watch` # or whatever app is using Listen

I have no idea if it's worth putting up a wiki with this info - I'd say it would be up to the authors/maintainers of the software (e.g. sass) to explain how to run their stuff with bundler.