Update README.md

Co-Authored-By: Bjørn Inge Hanssen <bihanssen@users.noreply.github.com>

README.md

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-*nRF Connect Direct Test Mode* is a cross-platform tool that lets you perform RF PHY testing of Bluetooth Low Energy devices with the Bluetooth specified Direct Test Mode.
+*nRF Connect Direct Test Mode* lets you perform RF PHY testing of Bluetooth Low Energy devices with the Bluetooth specified Direct Test Mode. See the [Bluetooth Core specification](https://www.bluetooth.com/specifications/bluetooth-core-specification/) (volume 6, part F) for more information on the Direct Test Mode specification.
 
 ![screenshot](resources/screenshot.gif)