Add SunFounder PiPower 3 overlay to README

Author: cavonlee

arch/arm/boot/dts/overlays/README

@@ -4701,6 +4701,10 @@ Load:   dtoverlay=sunfounder-pironman5,<param>=<val>
 Params: ir                      Enable IR or not (on or off, default on)
         ir_pins                 Change IR receiver pin (default 13)
 
+Name:   sunfounder-pipower3
+Info:   Overlay for SunFounder PiPower 3
+Load:   dtoverlay=sunfounder-pipower3,<param>=<val>
+Params: poweroff_pin            Change poweroff pin (default 26)
 
 Name:   superaudioboard
 Info:   Configures the SuperAudioBoard sound card