Add initial README with some setup and runner instructions #801

Job	Run time
Test Browser on macOS (safari, main)	50s
Test Build	27s
Test Browser on macOS (safari, disabled)	3m 8s
Test Browser on Linux (firefox, main)	46s
Test Browser on Linux (chrome, main)	1m 4s
Test Browser on Linux (firefox, default)	2m 27s
Test Browser on macOS (safari, default)	1m 19s
Test Browser on Linux (chrome, disabled)	1m 10s
Test Browser on Linux (chrome, default)	1m 12s
Test Shell (jsc, disabled)	1m 59s
Test Browser on Linux (firefox, disabled)	1m 37s
Test Shell (spidermonkey, main)	53s
Test Shell (v8, default)	2m 43s
Test Shell (v8, disabled)	1m 38s
Test Shell (spidermonkey, default)	5m 4s
Test Shell (v8, main)	40s
Test Shell (spidermonkey, disabled)	1m 51s
Test Shell (jsc, main)	44s
Test Shell (jsc, default)	3m 11s
	32m 43s

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