Build passing status icon

[Pjotr Prins]

Would it be possible to get a status icon straight from the build
farm, such as Travis CI provides, for Guix builds? That would mean we
could add it to a package README, e.g.

  https://github.com/pjotrp/bioruby-vcf/blob/master/README.md

The embedded markdown code snippet looks like

  [![Build 
Status](https://secure.travis-ci.org/pjotrp/bioruby-vcf.png)](http://travis-ci.org/pjotrp/bioruby-vcf)
 
When the build fails the PNG will be different.

Pj.