Enable host toolchain on macOS by default, improve `README.md` #20

Job	Run time
Soundness / Broken symlinks check	2s
Soundness / Broken symlinks check	2s
Soundness / Documentation check	34s
Soundness / Documentation check	37s
Soundness / API breakage check	0s
Soundness / API breakage check	0s
Soundness / Format check	36s
Soundness / Format check	32s
Soundness / Unacceptable language check	3s
Soundness / Unacceptable language check	3s
Soundness / License headers check	3s
Soundness / License headers check	5s
Soundness / Python lint check	4s
Soundness / Python lint check	4s
Soundness / Shell check	30s
Soundness / Shell check	33s
Soundness / YAML lint check	5s
Soundness / YAML lint check	3s
	3m 56s

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