Update README.md

Author: pdp7

README.md

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 # Adafruit BeagleBone I/O Python Library (Adafruit_BBIO)
 
-* Adafruit_BBIO is a set of Python tools to allow GPIO, PWM, and ADC access on the BeagleBone
+* Adafruit_BBIO is a set of Python tools to allow [GPIO](README.md#gpio), [PWM](README.md#pwm), [ADC](README.md#adc) and [UART](README.md#uart) access on the BeagleBone
 
 * It is recommended to use an [official BeagleBoard.org Debian image](https://beagleboard.org/latest-images)
   * **Currently recommended image: [Debian 9.2 "Stretch" iot (2017-10-29)](https://elinux.org/Beagleboard:BeagleBoneBlack_Debian#microSD.2FStandalone:_.28stretch-iot.29_.28All_BeagleBone_Variants_.26_PocketBeagle.29)**