Edit Readme for Ease of Use

[samiam2013]

The readme does a good job of explaining the use of enumer, comprehensive enough that you might not need more, but the actual use of the package is sort of below-the-fold or below what would be my first scroll. I'd like to add a very succinct example and the simplest instructions to install at the very start without removing any of the existing explanation(s). This would hopefully lower the bar for new and returning devs.