Improve README key features & add collapsed What-is-Nix section #536

Job	Run time
build (nix-snapshotter)	27s
lint (golangci-lint)	1m 4s
build (image-hello)	20s
build (image-redis)	29s
build (image-redisWithShell)	28s
integration-test (snapshotter)	4m 2s
integration-test (kubernetes)	4m 37s
integration-test (k3s)	13m 1s
integration-test (k3s-external)	14m 53s
integration-test (k3s-rootless)	12m 50s
coverage-report	44s
	52m 55s

Provide feedback