docs(readme): add #[OpenApiSpec] attribute usage documentation

Author: wadakatu

README.md

@@ -106,22 +106,51 @@ class GetPetsTest extends TestCase
 }
 ```
 
-To use a different spec for a specific test class, override `openApiSpec()`:
+To use a different spec for a specific test class, add the `#[OpenApiSpec]` attribute:
 
 ```php
+use Studio\OpenApiContractTesting\Laravel\OpenApiSpec;
+use Studio\OpenApiContractTesting\Laravel\ValidatesOpenApiSchema;
+
+#[OpenApiSpec('admin')]
 class AdminGetUsersTest extends TestCase
 {
     use ValidatesOpenApiSchema;
 
-    protected function openApiSpec(): string
+    // All tests in this class use the 'admin' spec
+}
+```
+
+You can also specify the spec per test method. Method-level attributes take priority over class-level:
+
+```php
+#[OpenApiSpec('front')]
+class MixedApiTest extends TestCase
+{
+    use ValidatesOpenApiSchema;
+
+    public function test_front_endpoint(): void
     {
-        return 'admin';
+        // Uses 'front' from class-level attribute
     }
 
-    // ...
+    #[OpenApiSpec('admin')]
+    public function test_admin_endpoint(): void
+    {
+        // Uses 'admin' from method-level attribute (overrides class)
+    }
 }
 ```
 
+Resolution priority (highest to lowest):
+
+1. Method-level `#[OpenApiSpec]` attribute
+2. Class-level `#[OpenApiSpec]` attribute
+3. `openApiSpec()` method override
+4. `config('openapi-contract-testing.default_spec')`
+
+> **Note:** You can still override `openApiSpec()` as before — it remains fully backward-compatible.
+
 #### Framework-agnostic
 
 ```php