Enable host toolchain on macOS by default, improve `README.md` #112

Job	Run time
Test / Linux (5.10 - jammy)	2m 42s
Test / Linux (6.0 - jammy)	2m 58s
Test / Linux (6.1 - jammy)	2m 58s
Test / Linux (nightly-main - jammy)	3m 34s
Test / Linux (nightly-6.2 - jammy)	3m 37s
Test / Linux (5.9 - jammy)	2m 36s
Soundness / Broken symlinks check	4s
Soundness / Unacceptable language check	4s
Test / macOS (Xcode ${{ matrix.xcode_version }} - ${{ matrix.os_version }} - ${{ matrix.arch }})	0s
Soundness / Format check	35s
Soundness / Documentation check	35s
Soundness / YAML lint check	6s
Soundness / Shell check	35s
Soundness / License headers check	4s
Soundness / Python lint check	7s
Test / Windows (${{ matrix.swift_version }} - windows-2022)	0s
Soundness / API breakage check	0s
	20m 35s

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