added instructions for finding Meta key on MacOS #467

Job	Run time
build-book	1m 45s
build-microbit-chapter (05-led-roulette)	40s
build-f3discovery-doc	1m 10s
build-f3discovery-chapter (05-led-roulette)	33s
build-microbit-chapter (07-uart)	43s
build-microbit-doc	2m 15s
build-f3discovery-chapter (06-hello-world)	29s
build-microbit-chapter (08-i2c)	38s
build-f3discovery-chapter (07-registers)	28s
build-microbit-chapter (09-led-compass)	39s
build-f3discovery-chapter (08-leds-again)	28s
build-microbit-chapter (10-punch-o-meter)	38s
build-f3discovery-chapter (09-clocks-and-timers)	30s
build-f3discovery-chapter (11-usart)	35s
build-f3discovery-chapter (14-i2c)	32s
build-f3discovery-chapter (15-led-compass)	28s
build-f3discovery-chapter (16-punch-o-meter)	28s
	12m 59s

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