adds led control functions to XD75 layout

keyboards/xd75/readme.md

@@ -26,6 +26,46 @@ firmware likely requires a command line like:
 $ sudo make xd75:default:dfu
 ```
 
+### LED control
+
+There are 3 individual LEDs that can be turned on and off, plus the keycap LEDs (which are all wired into the same pin).  The functions are named according to how they're labeled on the PCB.
+
+TODO: it would be nice to have PWM support on these LEDs for fade-in/fade-out effects.
+
+```c
+capslock_led_on();
+gp100_led_on();
+gp103_led_on();
+keycaps_led_on();
+
+
+// led_set_user example - you could also turn these on/off in response
+// to events in process_record_user or matrix_scan_user
+void led_set_user(uint8_t usb_led) {
+    if (usb_led & (1<<USB_LED_CAPS_LOCK)) {
+        capslock_led_on();
+    } else {
+        capslock_led_off();
+    }
+
+    if (some_custom_state) {
+      gp100_led_on();
+    }
+    else {
+      gp100_led_off();
+    }
+}
+```
+
+For the curious:
+
+```
+CAPSLOCK_LED    B2
+GP103_LED       F4
+KEYCAPS_LED     F5
+GP100_LED       F7
+```
+
 ### Other Keymaps
 
 The "default" keymap included is basically the OLKB Atomic keymap with