Ported docs on configuring Nagios from Metricinga to the NagiosPerfdataCollector.
Merge pull request #495 from craigcalef/nagiosperfdatadoc

Ported docs on configuring Nagios from Metricinga to the NagiosPerfdataCollector.  Originally from jgoldschrafe/metricinga
Merge branch 'master' of https://github.com/BrightcoveOS/Diamond