Add write-up of Piers' Intro to K8s talk that he delivered to the MVP meeting in July 2020.

Part of the work on PNET-91, to improve information for new people working on SKA.

This is a WIP, so I can get some feedback while I go through and add some more links, fix typos etc. I think it would be enhanced by a few diagrams, so if someone wishes to draw those, it would be much appreciated. We can either keep this as a WIP till we've got diagrams, or push what we've got, and add diagrams later.

The style is deliberately informal, as it was derived from a talk, and also it makes it a friendlier intro.