Describe timer sample rate in metric_types docs. Fixes #435

Code does accept a sample rate for time metrics:

https://github.com/etsy/statsd/blob/bad366cd438cd59b2064dd03ff3d415fc7db0c4b/stats.js#L206-L225

At least one client sends these (Openstack Swift) and at least one
server implementation considers them malformed currently (collectd),
presumbaly due to the bad docs :)