Collectd could not find plugin rrdtool

This is an annoying gotcha that I keep meaning to document. When installing collectd and rrdtool via yum on Centos 6/7, I always miss out the rrdtool lib. This manifests as

Starting collectd: Could not find plugin rrdtool.

when restarting the services.
Thankfully it is easy to fix. Run

yum install collectd-rrdtool

and restart the collectd service and it should start without complaint.