Add docs badge to README

Hi there,

you tried to take a look at your project on http://inch-ci.org but that did not work because of a routing error. It does work after a small [patch](https://github.com/inch-ci/inch_ci-web/commit/5127fc6112575ae82f99bd13f7d1b78a79a5c73d) to the service.

This badge is the result: http://inch-ci.org/github/biow0lf/prometheus2.0