Update and expand comparison doc. (#666)
Update and expand comparison doc. (#666)

Add Nagios and Sensu, update existing ones.
chosenken/docs,chosenken/docs,chosenken/docs,prometheus/docs,ddfreyne/prometheus-docs,du2016/prometheus-docs,ddfreyne/prometheus-docs,open-falcon/open-falcon.github.io,Cepave/openfalcon.io,ddfreyne/prometheus-docs,du2016/prometheus-docs,chosenken/docs,prometheus/docs,open-falcon/open-falcon.github.io,ddfreyne/prometheus-docs,prometheus/docs,Cepave/openfalcon.io,du2016/prometheus-docs,open-falcon/open-falcon.github.io,Cepave/openfalcon.io