Update README with response signing documentation

- Added Response Signing section explaining X-License-Signature and X-License-Timestamp headers
- Added wp-config.php configuration example for WC_LICENSE_SERVER_SECRET
- Updated client section to recommend official magdev/wc-licensed-product-client Composer package
- Documented LicenseClient and SecureLicenseClient classes

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -107,12 +107,42 @@ When a customer purchases a licensed product, they must enter the domain where t
 
 Full API documentation available in `openapi.json` (OpenAPI 3.1 specification).
 
-### Client Examples
+### Response Signing (Optional)
 
-Ready-to-use API client examples are available in `docs/client-examples/`:
+When the server is configured with a shared secret, all API responses include cryptographic signatures for tamper protection:
+
+**Configuration (wp-config.php):**
+
+```php
+define('WC_LICENSE_SERVER_SECRET', 'your-secure-random-string-min-32-chars');
+```
+
+**Response Headers:**
+
+| Header | Description |
+| ------ | ----------- |
+| `X-License-Signature` | HMAC-SHA256 signature of the response body |
+| `X-License-Timestamp` | Unix timestamp when the response was generated |
+
+The signature prevents man-in-the-middle attacks and ensures response integrity. Use the `magdev/wc-licensed-product-client` Composer package with the `SecureLicenseClient` class to automatically verify signatures.
+
+### Client Libraries & Examples
+
+**PHP (Recommended):** Install the official client library via Composer:
+
+```bash
+composer require magdev/wc-licensed-product-client
+```
+
+The library provides:
+
+- `LicenseClient` - Standard client for API calls
+- `SecureLicenseClient` - Client with automatic response signature verification
+
+**Example clients** for other languages are available in `docs/client-examples/`:
 
 - **cURL** - Shell script examples ([curl.sh](docs/client-examples/curl.sh))
-- **PHP** - Client class with examples ([php-client.php](docs/client-examples/php-client.php))
+- **PHP** - Standalone client example ([php-client.php](docs/client-examples/php-client.php))
 - **Python** - Client class with dataclasses ([python-client.py](docs/client-examples/python-client.py))
 - **JavaScript** - Browser and Node.js client ([javascript-client.js](docs/client-examples/javascript-client.js))
 - **C#** - Async client with System.Text.Json ([csharp-client.cs](docs/client-examples/csharp-client.cs))