Document how to write a new ipdevpoll plugin

[lunkwill42]

Is your feature request related to a problem? Please describe.

There doesn't seem to be any proper guide to writing a new ipdevpoll plugin. There is only a howto guide for writing new MibRetriever implementations to imbue the existing sensors plugin with the capability to collect sensor inventory from new vendors/MIBS.

Describe the solution you'd like

A proper howto guide that explains how to construct a relatively simple ipdevpoll plugin and add it to an existing job.