Augment README.md in top level to indicate Makefile presence #1126

Job	Run time
compiler (gcc-7, ubuntu-20.04)	4m 1s
compiler (gcc-8, ubuntu-20.04)	4m 3s
compiler (gcc-9, ubuntu-20.04)	4m 12s
compiler (gcc-10, ubuntu-20.04)	3m 58s
compiler (gcc-11, ubuntu-22.04)	4m 26s
compiler (gcc-12, ubuntu-22.04)	4m 54s
compiler (gcc-13, ubuntu-22.04, ubuntu-toolchain-r/test)	4m 44s
compiler (clang-6.0, ubuntu-20.04)	4m 23s
compiler (clang-7, ubuntu-20.04)	4m 15s
compiler (clang-8, ubuntu-20.04)	4m 24s
compiler (clang-9, ubuntu-20.04)	4m 34s
compiler (clang-10, ubuntu-20.04)	4m 26s
compiler (clang-11, ubuntu-20.04)	4m 23s
compiler (clang-12, ubuntu-20.04)	4m 22s
compiler (clang-13, ubuntu-22.04)	5m 0s
compiler (clang-14, ubuntu-22.04)	5m 3s
compiler (clang-15, ubuntu-22.04, jammy)	4m 58s
compiler (clang-16, ubuntu-22.04, jammy)	5m 3s
compiler (clang-17, ubuntu-22.04, jammy)	5m 8s
	1h 26m 17s

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