Documentation on code coverage

I updated the README to explain how to write code coverage reports with grunt-testem.

This PR depends on sideroad/testem-multi#6. 
Once that PR is merged and the new version is published in npm, this can work.

There's also an example on how to configure grunt + istanbul to instrument and generate a report.

I've marked this version as `0.5.0` because now `grunt-testem` depends on `testem-multi` 0.4.x and not anymore on 0.3.x but there's no backward incompatible change, so if you prefer we can call it `0.4.1`