Test External

Add `readme.md` to explain usage. #382

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Run time

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Job	Run time
3.1 on ubuntu	46s
3.2 on ubuntu	45s
3.3 on ubuntu	43s
3.4 on ubuntu	49s
3.1 on macos	34s
3.2 on macos	30s
3.3 on macos	33s
3.4 on macos	32s
	5m 12s