added instructions for finding Meta key on MacOS #466

Job	Run time
build-f3discovery-doc	1m 7s
build-book	1m 49s
build-f3discovery-chapter (05-led-roulette)	35s
build-microbit-chapter (05-led-roulette)	38s
build-f3discovery-chapter (06-hello-world)	28s
build-microbit-doc	2m 18s
build-microbit-chapter (07-uart)	36s
build-f3discovery-chapter (07-registers)	30s
build-microbit-chapter (08-i2c)	37s
build-f3discovery-chapter (08-leds-again)	30s
build-microbit-chapter (09-led-compass)	41s
build-f3discovery-chapter (09-clocks-and-timers)	30s
build-microbit-chapter (10-punch-o-meter)	37s
build-f3discovery-chapter (11-usart)	32s
build-f3discovery-chapter (14-i2c)	28s
build-f3discovery-chapter (15-led-compass)	28s
build-f3discovery-chapter (16-punch-o-meter)	29s
	12m 53s

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