bluetooth: samples: hci_uart: Add Direction Finding doc
bluetooth: samples: hci_uart: Add Direction Finding doc

Add the Direction Finding documentation section into
the sample README.rst file.

Signed-off-by: Kamil Gawor <134c1a8a460515c422169a7c5be0d402d35097d9@nordicsemi.no>

galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr