Document how to create a threadsafe registry

[kevinburkeshyp]

My first iteration of a metrics client had code like this:

// Increment a counter with the given name.
func Increment(name string) {
    mn := getWithNamespace(name)
    m := metrics.GetOrRegister(mn, metrics.NewMeter()).(metrics.Meter)
    m.Mark(1)
}

Which contained a massive resource leak - every anonymous NewMeter() call appends a new arbiter, so I had one arbiter for each function call! Instead document the right way to do this so people hopefully don't get as lost.

[mihasya]

Can't argue with more docs, though unfortunately this needs a rebase now :| sorry about my slowness.

[kevinburkeshyp]

Fixed conflict in #186 (I can't push to the branch in this PR anymore)