Include configuration schema in the README

Author: puddly

README.md

@@ -25,6 +25,19 @@ custom_zha_radios:
    description: TI CC13x2, CC26x2, and ZZH
 ```
 
+# Configuration
+
+Below are the defaults with the top-level Home Assistant `zha:` key:
+
+```yaml
+zha:
+  zigpy_config:
+    znp_config:
+      tx_power: None           # if set, must be between -22 (low) and 19 (high)
+      led_mode: None           # if set, must be one of: off, on, blink, flash, toggle
+      skip_bootloader: True    # skips the 60s bootloader delay on some CC2531 sticks
+```
+
 # Backup and restore
 A complete NVRAM backup can be performed to migrate between different radios **based on the same chip**. Anything else is untested.