Add example "pretty output" to the `README.md` file

[adarsh]

If the subtitle is "A parallel test runner for RSpec and Cucumber with pretty output", it would be nice to see an example of that output in the README.md file. :)

[WaKeMaTTa]

You lazy boy!

I can testify that is true

[briandunn]

The output is pretty much identical to that of rspec -fprogress. Even more so on 2.0. A pr with a gif would be most welcome though!