Clarify library intro in README #1929

Job	Run time
build-linux (gcc)	1m 5s
build-linux-bazel	6m 30s
build-linux (clang)	1m 1s
build-macos (gcc)	1m 13s
build-windows (msvc)	2m 18s
build-macos (clang)	46s
build-windows (clang)	1m 40s
build-windows (gcc)	2m 27s
build-linux-all-configurations (gcc)	9m 3s
build-linux-all-configurations (clang)	9m 26s
build-macos-all-configurations (gcc)	4m 41s
build-macos-all-configurations (clang)	4m 21s
build-windows-all-configurations (msvc)	4m 19s
build-windows-all-configurations (clang)	4m 37s
build-windows-all-configurations (gcc)	7m 46s
	1h 1m 13s

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