Add CNCF reference in the README

A simple reference to the CNCF and link to the website.

README.md

@@ -7,7 +7,7 @@
 Visit [prometheus.io](https://prometheus.io) for the full documentation,
 examples and guides.
 
-Prometheus is a systems and service monitoring system. It collects metrics
+Prometheus, a [Cloud Native Computing Foundation](https://cncf.io/) project, is a systems and service monitoring system. It collects metrics
 from configured targets at given intervals, evaluates rule expressions,
 displays the results, and can trigger alerts if some condition is observed
 to be true.