Link to Prometheus project in the README

[sbeitzel]

It'd be helpful to people coming by this project to provide a little bit of context about what Prometheus is, that one might want to have a client for it, Swift or otherwise. The fix for this could be a simple one-liner in the README, along the lines of:

A Swift client for the [Prometheus](https://github.com/prometheus/prometheus) monitoring system,
supporting counters, gauges and histograms.