docs(readme): add Cluster Broadcast section and link to detailed guide

Documents centralized cluster broadcasting flow: service fires local event, core sends UDP CLUSTER_EVENT, peers re-fire locally. Includes usage example and notes; links to docs/ClusterBroadcast.md.

README.md

@@ -9,6 +9,7 @@ SPORE is a cluster engine for ESP8266 microcontrollers that provides automatic n
 - [Features](#features)
 - [Supported Hardware](#supported-hardware)
 - [Architecture](#architecture)
+- [Cluster Broadcast](#cluster-broadcast)
 - [API Reference](#api-reference)
 - [Configuration](#configuration)
 - [Development](#development)
@@ -26,6 +27,7 @@ SPORE is a cluster engine for ESP8266 microcontrollers that provides automatic n
 - **Service Registry**: Dynamic API endpoint discovery and registration
 - **Health Monitoring**: Real-time node status tracking with resource monitoring
 - **Event System**: Local and cluster-wide event publishing/subscription
+- **Cluster Broadcast**: Centralized UDP broadcast of events (CLUSTER_EVENT)
 - **Over-The-Air Updates**: Seamless firmware updates across the cluster
 - **REST API**: HTTP-based cluster management and monitoring
 - **Capability Discovery**: Automatic API endpoint and service capability detection
@@ -103,6 +105,35 @@ void setup() {
 
 **Examples:** See [`examples/base/`](./examples/base/) for basic usage and [`examples/relay/`](./examples/relay/) for custom service integration.
 
+## Cluster Broadcast
+
+Broadcast an event to all peers using the centralized core broadcaster. Services never touch UDP directly; instead they fire a local event that the core transmits as a `CLUSTER_EVENT`.
+
+Usage:
+
+```cpp
+// 1) Apply locally via the same event your service already handles
+JsonDocument payload;
+payload["pattern"] = "rainbow_cycle";
+payload["brightness"] = 100;
+String payloadStr; serializeJson(payload, payloadStr);
+ctx.fire("api/neopattern", &payloadStr);
+
+// 2) Broadcast to peers via the core
+JsonDocument envelope;
+envelope["event"] = "api/neopattern";
+envelope["data"] = payloadStr; // JSON string
+String eventJson; serializeJson(envelope, eventJson);
+ctx.fire("cluster/broadcast", &eventJson);
+```
+
+Notes:
+- The core sends subnet-directed broadcasts (e.g., 192.168.1.255) for reliability.
+- Peers receive `CLUSTER_EVENT` and forward to local subscribers with `ctx.fire(event, data)`.
+- `data` can be a JSON string or nested JSON; receivers handle both.
+
+📖 See the dedicated guide: [`docs/ClusterBroadcast.md`](./docs/ClusterBroadcast.md)
+
 ## API Reference
 
 The system provides a comprehensive RESTful API for monitoring and controlling the embedded device. All endpoints return JSON responses and support standard HTTP status codes.