Put more details about portable in readme. (#11816) #510

Job	Run time
test-unix-nightly (linux, 3.11, 12.1, nightly)	1d 0h 0m 0s
test-stable (linux, 3.10, 12.1, stable)	1s
test-stable (linux, 3.11, 12.1, stable)	1s
test-stable (linux, 3.12, 12.1, stable)	1s
	1d 0h 0m 3s

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