boards/anavi/knobs3/README.md: Display

Add information about the Display extension for ANAVI Knobs 3.

Signed-off-by: Leon Anavi <leon@anavi.org>

Author: leon-anavi

boards/anavi/knobs3/README.md

@@ -8,3 +8,4 @@ Extensions enabled by default:
 - [Encoder](/docs/en/encoder.md) Twist control for all the things
 - [RGB](/docs/en/rgb.md) Light it up (for underlighting)
 - [MediaKeys](/docs/en/media_keys.md) Control volume and other media functions
+- [Display](/docs/en/Display.md) Show information on the mini OLED display