docs: explain SA automounting & autoconf
Add a few words about how the client automatically attempts to pick up
authentication credentials and other information about a cluster
directly from Kubernetes, and what to do if that's not what you want.
This is a follow-up to this conversation on gitter:
https://matrix.to/#/!rDlRZKYupRPQnkjTvm:gitter.im/$xrYokOTFaI1N4Q1xRSHMGlJ5v68VGy-piWTXuFrFmJ4?via=gitter.im&via=matrix.org
With many thanks to Rohan Kumar.
