The relative links don't work in other pages that render the README (for example https://hub.docker.com/r/prom/prometheus/). As they are (hopefully) not due to change any time soon, I think using absolute links is better.

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

building tool

For building from source as an end user running the tests is not necessary.

It's almost at an end, so we can remove this.

Fixes https://github.com/prometheus/prometheus/issues/1165

As DigitalOcean is linking to this page, direct users to the project list from here.