Expand documentation of mapping

[matthiasr]

Breakout from #80:

• Provide examples using regex not just glob in the documentation
• Provide an example using a catch-all regex for metrics not matching an existing regex.
• Explain the difference between using the configuration for dropping all non match metrics versus using a catch all regex to provide an indication that metrics are not matching and would have been dropped.

The mapping is closely related to that of the statsd exporter – the main difference is that all Graphite metrics are considered a gauge.

Maybe it makes sense to have a unified mapping documentation, but for now, the statsd exporter readme can serve as an inspiration for this one.