doc: add README.md to the example

Author: trombik

examples/default/README.md

@@ -0,0 +1,11 @@
+## What the example does
+
+* Prints received events from the encoder such as "Button pressed" and the
+  position value in a loop.
+
+## Wiring
+
+| Chip    | A  | B  | Button |
+|---------|----|----|--------|
+| ESP8266 | 14 | 12 | 13     |
+| ESP32s  | 16 | 17 | 5      |