Document syncing layer change callback across halves

docs/feature_layers.md

@@ -179,6 +179,44 @@ Use the `IS_LAYER_ON_STATE(state, layer)` and `IS_LAYER_OFF_STATE(state, layer)`
 
 Outside of `layer_state_set_*` functions, you can use the `IS_LAYER_ON(layer)` and `IS_LAYER_OFF(layer)` macros to check global layer state.
 
+### Running on slave half as well as master (for split keyboards)
+
+By default `layer_state_set_*` functions only run on master. A workaround for this is to use split custom communication to call the slave half.
+
+In `keymap.c`:
+
+```c
+#include "transactions.h"
+
+void update_layer_state_set(uint8_t state, const void* unused, uint8_t unused2, void* unused3) {
+    layer_state_set_user(state);
+}
+
+void keyboard_post_init_user(void) {
+    // Register callback to update layer LEDs on layer change.
+    transaction_register_rpc(SYNC_LAYER_CHANGE, update_layer_state_set);
+}
+
+layer_state_t layer_state_set_user(layer_state_t state) {
+    /* Other config code
+    ...
+    */
+    if (is_keyboard_master()){
+        transaction_rpc_exec(SYNC_LAYER_CHANGE, state, NULL, 0, NULL);
+    }
+return state;
+```
+
+In `config.h`:
+
+```c
+// Transaction ID for inter-half coms.
+#define SPLIT_TRANSACTION_IDS_USER SYNC_LAYER_CHANGE
+
+#define SPLIT_LAYER_STATE_ENABLE
+```
+
+
 ### `layer_state_set_*` Function Documentation
 
 * Keyboard/Revision: `layer_state_t layer_state_set_kb(layer_state_t state)`