Improve README key features & add collapsed What-is-Nix section #535

Job	Run time
lint (golangci-lint)	1m 6s
build (nix-snapshotter)	21s
build (image-hello)	30s
build (image-redis)	22s
build (image-redisWithShell)	28s
integration-test (snapshotter)	4m 2s
integration-test (kubernetes)	4m 24s
integration-test (k3s)	13m 19s
integration-test (k3s-external)	14m 27s
integration-test (k3s-rootless)	12m 50s
coverage-report	40s
	52m 29s

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