Add FAQ entry on rootful vs rootless and clarify where to apply overlay #540

Job	Run time
lint (golangci-lint)	59s
build (nix-snapshotter)	1m 28s
build (image-hello)	1m 28s
build (image-redis)	1m 31s
build (image-redisWithShell)	1m 40s
integration-test (snapshotter)	4m 52s
integration-test (kubernetes)	5m 38s
integration-test (k3s)	13m 32s
integration-test (k3s-external)	15m 27s
integration-test (k3s-rootless)	13m 52s
coverage-report	49s
	1h 1m 16s

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