Adding example demonstrating syntax for usage with Test::Unit to README.

It says in the very beginning of the README that the gem is compatible with `Test::Unit`, however, all successive code examples demonstrate usage in `Rspec`.  I thought it would be helpful to provide an example demonstrating syntax to use in `Test::Unit`.

I love this gem and use it frequently when working with `Rspec`, however, I recently took over  a project that had a fully fledged test suite written in `Test::Unit`.  Having little experience with `Test::Unit`, it took some experimentation before I was able to get the 'one-liners' working in `Test::Unit` that I had become so accustomed to using in `Rspec`.  I also struggled to find examples of usage in conjunction with `Test::Unit` anywhere in the documentation or on the web, (eventually I found [an issue filed against `shoulda` which pointed me in the right direction](https://github.com/thoughtbot/shoulda/issues/216)).  This type of thing would have been handy though.