Support badges and add them to the README

[wilzbach]

If we don't go with #138 (Only build on PRs and don't build on commits to stable or master), it probably makes sense to increase the visibility of the status and add a badge for it to all repos (or maybe start to create a dedicated CI overview page).

There's already a plugin which could probably be easily installed as a start:

https://plugins.jenkins.io/embeddable-build-status

[wilzbach]

Buildkite supports badges out of the box and it's already added here:

Once the pipeline is more stable, we could add the badges to the individual repos.