Add documentation for how to use grunt-istanbul

One thing that I got stuck on when implementing grunt-istanbul in my project is that I need to point my tests at my instrumented source code. I updated the readme to describe this process and added some example code. 

I also wrote a blog post about testing an express.js app and running code coverage using this grunt-istanbul plugin: http://www.gregjopa.com/2014/02/testing-and-code-coverage-with-node-js-apps/.
And here's a link to the sample app: https://github.com/gregjopa/express-app-testing-demo