Add initial README with some setup and runner instructions #799

Job	Run time
Test Browser on macOS (safari, disabled)	1m 43s
Test Browser on macOS (safari, main)	1m 15s
Test Shell (jsc, disabled)	1m 55s
Test Browser on macOS (safari, default)	1m 19s
Test Build	1m 11s
Test Browser on Linux (firefox, default)	1m 40s
Test Browser on Linux (chrome, disabled)	1m 8s
Test Browser on Linux (firefox, disabled)	1m 22s
Test Browser on Linux (firefox, main)	46s
Test Shell (jsc, main)	51s
Test Shell (v8, disabled)	1m 35s
Test Browser on Linux (chrome, default)	1m 19s
Test Shell (v8, default)	2m 38s
Test Browser on Linux (chrome, main)	1m 8s
Test Shell (v8, main)	46s
Test Shell (spidermonkey, disabled)	2m 1s
Test Shell (spidermonkey, main)	57s
Test Shell (jsc, default)	3m 9s
Test Shell (spidermonkey, default)	5m 19s
	32m 2s

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