Add release badges for non-default tracks

[agileshaw]

Since prometheus-juju-exporter snap is versioned corresponding to juju controller's version, in most cases users would want to install a versioned track instead of the default latest/stable. Therefore, having badges for multiple non-default track in README would be useful to show the release status of those tracks.

Unfortunately, snap store doesn't currently support badge generation for non-default tracks, so we would have to wait until this feature is implemented on their side.