Add reference to grape-librato gem to README

Hey there.

Based on the `newrelic-grape` gem, I wanted to get some similar functionality into my [Librato Metrics](http://librato.com) setup, where it automatically creates a metric for each Grape endpoint. If you're not familiar with Librato, they're a metrics/monitoring solution that's integrated with Heroku. 

The result is the [grape-librato](https://github.com/seanmoon/grape-librato) gem.

I added a reference to it in the README, if you'd rather leave it out, no worries. If you'd like it in, merge this in.

Thanks!

Sean