docs: add k8s provider docs to README #181

Job	Run time
Build concierge	34s
Define spread matrix	11s
Spread (extra-debs)	1m 5s
Spread (extra-packages-config-file)	1m 21s
Spread (extra-snaps)	56s
Spread (juju-model-defaults)	2m 30s
Spread (overrides-env)	5m 44s
Spread (overrides-priority)	47s
Spread (preset-dev)	5m 52s
Spread (preset-k8s)	3m 8s
Spread (preset-machine)	3m 9s
Spread (preset-microk8s)	5m 1s
Spread (provider-google)	58s
Spread (provider-k8s)	3m 23s
Spread (provider-lxd)	2m 42s
Spread (provider-lxd-init-no-bootstrap)	6m 2s
Spread (provider-microk8s)	3m 15s
Spread (provider-none)	37s
Spread (restore)	8m 3s
	55m 18s

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