Add README with setup, usage, and custom effect guide

README.md

@@ -0,0 +1,153 @@
+# zmk-rgb-effects
+
+Modular RGB effects for ZMK keyboards. Provides per-key per-layer RGB indicators and a framework for adding custom effects without forking ZMK.
+
+Requires: [ZMK fork](https://github.com/afiqzudinhadi/zmk) branch `rgb-modular-v1.1` (modular effect registration API).
+
+## Setup
+
+Add to your `west.yml`:
+
+```yaml
+manifest:
+  remotes:
+    - name: afiqzudinhadi
+      url-base: https://github.com/afiqzudinhadi
+  projects:
+    - name: zmk
+      remote: afiqzudinhadi
+      revision: rgb-modular-v1.1
+      import: app/west.yml
+    - name: zmk-rgb-effects
+      remote: afiqzudinhadi
+      revision: v1.1.1
+```
+
+Enable in your `.conf`:
+
+```
+CONFIG_ZMK_RGB_UNDERGLOW=y
+CONFIG_ZMK_RGB_EFFECT_LAYER=y
+```
+
+## Per-key layer effect
+
+Define per-key colors in your keymap/overlay using devicetree:
+
+```dts
+#include <dt-bindings/zmk/rgb_colors.h>
+
+/ {
+    underglow-layer {
+        compatible = "zmk,underglow-layer";
+        pixel-lookup = <
+            5  4  3  2  1  0
+            6  7  8  9  10 11
+            17 16 15 14 13 12
+        >;
+
+        layer0 {
+            layer-id = <0>;
+            bindings = <
+                &ugc RGB_RED  &ugc RGB_RED  &ugc RGB_OFF ...
+            >;
+        };
+
+        layer1 {
+            layer-id = <1>;
+            fade-delay = <15>;
+            bindings = <
+                &ugc RGB_BLUE &ugc RGB_BLUE &ugc RGB_OFF ...
+            >;
+        };
+    };
+};
+```
+
+### Properties
+
+- `pixel-lookup` — maps LED chain index to key position
+- `layer-id` — which keymap layer this color map applies to
+- `fade-delay` — seconds before reverting to animated effect (-1 = never)
+- `bindings` — color per key using behaviors below
+
+### Behaviors
+
+| Behavior | Description | Params |
+|----------|-------------|--------|
+| `&ugc` | Static color | `param1` = RGB hex color |
+| `&ubi` | Battery indicator | `param1` = low color, `param2` = ok color, `threshold` DT prop |
+| `&ugi` | HID indicator (caps/num/scroll lock) | `param1` = off color, `param2` = on color, `indicator` DT prop |
+
+## Adding custom effects
+
+Create a `.c` file with `ZMK_RGB_EFFECT_DEFINE`:
+
+```c
+#include <zmk/rgb_effect.h>
+
+static void my_render(struct zmk_rgb_effect_ctx *ctx) {
+    for (int i = 0; i < ctx->num_pixels; i++) {
+        struct zmk_led_hsb hsb = ctx->base_color;
+        hsb.h = (*ctx->animation_step + i * 30) % 360;
+        ctx->pixels[i] = zmk_rgb_hsb_to_rgb(zmk_rgb_hsb_scale_min_max(hsb));
+    }
+    *ctx->animation_step += ctx->animation_speed;
+}
+
+ZMK_RGB_EFFECT_DEFINE(my_effect, "My Effect",
+    my_render, 0, NULL, NULL, NULL, NULL);
+```
+
+Add to `CMakeLists.txt`:
+
+```cmake
+target_sources(app PRIVATE src/effects/my_effect.c)
+```
+
+The effect auto-registers via linker section. Accessible by cycling with `RGB_EFF`.
+
+### Effect flags
+
+| Flag | Description |
+|------|-------------|
+| `0` | Animated — core runs render at 25ms tick |
+| `ZMK_RGB_EFFECT_STATIC` | No periodic tick — render called on demand via `zmk_rgb_request_refresh()` |
+| `ZMK_RGB_EFFECT_PERSISTENT` | Survives RGB toggle and idle sleep |
+
+### Effect callbacks
+
+| Callback | When called |
+|----------|-------------|
+| `render` | Each tick (animated) or on refresh (static) |
+| `on_select` | Effect becomes active |
+| `on_deselect` | User cycles to different effect |
+| `on_idle(bool awake)` | Keyboard sleeps/wakes (persistent effects only) |
+| `is_active()` | Queried for ext power gating — return `true` to keep LEDs powered |
+
+### Render context
+
+| Field | Type | Description |
+|-------|------|-------------|
+| `pixels` | `struct led_rgb *` | LED buffer to write |
+| `num_pixels` | `uint16_t` | Strip length |
+| `base_color` | `struct zmk_led_hsb` | User-configured HSB color |
+| `animation_step` | `uint16_t *` | Read/write animation counter |
+| `animation_speed` | `uint8_t` | User-configured speed (1-5) |
+
+### Utility functions
+
+- `zmk_rgb_hsb_to_rgb(hsb)` — HSB to RGB conversion
+- `zmk_rgb_hsb_scale_min_max(hsb)` — scale brightness to configured min/max
+- `zmk_rgb_hsb_scale_zero_max(hsb)` — scale brightness from 0 to max
+- `zmk_rgb_request_refresh()` — trigger re-render for static effects
+- `zmk_rgb_request_refresh_wakeup(bool)` — refresh with conditional wake from sleep
+- `zmk_rgb_set_tick_delay(int seconds)` — delayed tick restart (for fade effects)
+- `zmk_rgb_is_on()` — query RGB on/off state
+
+## Versioning
+
+`vX.Y.Z` where:
+- **X** — major version (breaking API changes)
+- **Y** — ZMK core dependency version (matches `rgb-modular-vX.Y`)
+- **Z** — custom effects (0 = defaults only, 1+ = custom effects included)