doc: Mention pkg-config in README #1177

Job	Run time
build-ubuntu-latest-minimal-gcc	45s
build-ubuntu-latest-full-featured-gcc	1m 25s
build-ubuntu-latest-clang-analyzer	1m 8s
build-ubuntu-latest-clang-sanitizer	57s
build-ubuntu-latest-full-featured-clang	1m 29s
build-ubuntu-latest-gcc-static	56s
build-ubuntu-latest-minimal-clang	1m 11s
build-ubuntu-latest-pcp	1m 49s
build-dragonflybsd-latest-gcc	4m 12s
build-freebsd-latest-clang	2m 53s
build-macos-latest-clang	38s
build-netbsd-latest-gcc	3m 58s
build-openbsd-latest-clang	4m 11s
build-solaris-latest-gcc	3m 43s
lint-whitespace	3s
lint-pcp-config	3s
	29m 21s

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