Extend prefetching/preloading section of the README #803

Job	Run time
Test Build	4m 37s
Test Browser on macOS (safari, default)	2m 34s
Test Browser on Linux (chrome, disabled)	1m 1s
Test Browser on Linux (firefox, main)	49s
Test Browser on macOS (safari, main)	1m 21s
Test Browser on macOS (safari, disabled)	1m 40s
Test Browser on Linux (firefox, disabled)	1m 31s
Test Shell (spidermonkey, disabled)	1m 50s
Test Browser on Linux (chrome, main)	34s
Test Shell (v8, main)	43s
Test Shell (jsc, main)	55s
Test Shell (spidermonkey, default)	5m 10s
Test Browser on Linux (chrome, default)	1m 45s
Test Shell (jsc, disabled)	1m 47s
Test Shell (jsc, default)	3m 19s
Test Browser on Linux (firefox, default)	1m 30s
Test Shell (spidermonkey, main)	45s
Test Shell (v8, disabled)	1m 53s
Test Shell (v8, default)	2m 48s
	36m 32s

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