added instructions for finding Meta key on MacOS #469

Job	Run time
build-f3discovery-doc	1m 7s
build-microbit-chapter (05-led-roulette)	39s
build-book	28s
build-microbit-chapter (07-uart)	37s
build-microbit-doc	2m 14s
build-microbit-chapter (08-i2c)	42s
build-f3discovery-chapter (05-led-roulette)	35s
build-microbit-chapter (09-led-compass)	40s
build-f3discovery-chapter (06-hello-world)	28s
build-microbit-chapter (10-punch-o-meter)	36s
build-f3discovery-chapter (07-registers)	26s
build-f3discovery-chapter (08-leds-again)	24s
build-f3discovery-chapter (09-clocks-and-timers)	29s
build-f3discovery-chapter (11-usart)	36s
build-f3discovery-chapter (14-i2c)	27s
build-f3discovery-chapter (15-led-compass)	29s
build-f3discovery-chapter (16-punch-o-meter)	23s
	11m 20s

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