docs: Add executor configuration section to README

Document the @internal executor configuration options and their impact on
streaming performance. Explains why the dedicated executor matters for
preventing ForkJoinPool saturation under concurrent load.

Includes:
- Default configuration values (15 core, 30 max threads)
- Property names for customization
- Why it matters for streaming performance
- Note about custom server integrations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kabir

README.md

@@ -232,6 +232,38 @@ public class WeatherAgentExecutorProducer {
 }
 ```
 
+### 4. Configure Executor Settings (Optional)
+
+The A2A Java SDK uses a dedicated executor for handling asynchronous operations like streaming subscriptions. By default, this executor is configured with 15 threads optimized for I/O-bound operations.
+
+You can customize the executor settings in your `application.properties`:
+
+```properties
+# Thread pool size for the @Internal executor (default: 15)
+a2a.internal.executor.core-pool-size=15
+
+# Maximum thread pool size (default: 30)
+a2a.internal.executor.max-pool-size=30
+
+# Thread keep-alive time in seconds (default: 60)
+a2a.internal.executor.keep-alive-time=60
+```
+
+**Why this matters:**
+- **Streaming Performance**: The executor handles streaming subscriptions. Too few threads can cause timeouts under concurrent load.
+- **Resource Management**: The dedicated executor prevents streaming operations from competing with the ForkJoinPool used by other async tasks.
+- **Concurrency**: In production environments with high concurrent streaming requests, increase the pool sizes accordingly.
+
+**Default Configuration:**
+```properties
+# These are the defaults - no need to set unless you want different values
+a2a.internal.executor.core-pool-size=15
+a2a.internal.executor.max-pool-size=30
+a2a.internal.executor.keep-alive-time=60
+```
+
+**Note:** The reference server implementations automatically configure this executor. If you're creating a custom server integration, ensure you provide an `@Internal Executor` bean for optimal streaming performance.
+
 ## A2A Client
 
 The A2A Java SDK provides a Java client implementation of the [Agent2Agent (A2A) Protocol](https://google-a2a.github.io/A2A), allowing communication with A2A servers. The Java client implementation supports the following transports: