Clarify library intro in README #1930

Job	Run time
build-linux-bazel	6m 15s
build-linux (gcc)	1m 0s
build-linux (clang)	1m 7s
build-macos (gcc)	48s
build-windows (msvc)	2m 5s
build-windows (clang)	2m 3s
build-windows (gcc)	2m 14s
build-macos (clang)	45s
build-macos-all-configurations (gcc)	4m 47s
build-macos-all-configurations (clang)	4m 41s
build-windows-all-configurations (msvc)	4m 54s
build-windows-all-configurations (clang)	4m 33s
build-windows-all-configurations (gcc)	7m 7s
build-linux-all-configurations (gcc)	8m 38s
build-linux-all-configurations (clang)	9m 25s
	1h 0m 22s

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