Add comprehensive client implementation documentation

- Create docs/client-implementation.md with full integration guide
- Document all classes: clients, DTOs, exceptions, security classes
- Add integration examples for WordPress, Laravel, Symfony
- Include best practices and troubleshooting sections
- Update README with documentation links

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

README.md

@@ -138,6 +138,13 @@ try {
 - **Per-License Keys**: Each license has a unique verification key
 - **Code Integrity**: Optional verification of source file integrity
 
+## Documentation
+
+For detailed implementation guides, see:
+
+- [Client Implementation Guide](docs/client-implementation.md) - Complete guide for integrating this client into existing projects
+- [Server Implementation Guide](docs/server-implementation.md) - How to set up response signing on the server
+
 ## Testing
 
 Run the test suite with PHPUnit: