docs: Add OAuth2 configuration example to README

- Add README.md to .openapi-generator-ignore to preserve custom docs
- Include proper OAuth2ClientHttpRequestInterceptor configuration example
- Document Spring Boot 3.5+ / Spring Security 6.5+ requirements

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

Author: ng-galien

samples/client/petstore/spring-http-interface-oauth/.openapi-generator-ignore

@@ -21,3 +21,6 @@
 #docs/*.md
 # Then explicitly reverse the ignore rule for a single file:
 #!docs/README.md
+
+# Preserve custom OAuth2 documentation
+README.md

samples/client/petstore/spring-http-interface-oauth/README.md

@@ -1,21 +1,108 @@
-# OpenAPI generated API stub
-
-[Spring Framework 6 HTTP Interface](https://docs.spring.io/spring-framework/docs/6.0.0/reference/html/integration.html#rest-http-interface)
+# Spring HTTP Interface with OAuth2 (@ClientRegistrationId)
 
+This sample demonstrates the use of the `@ClientRegistrationId` annotation with Spring HTTP Interface clients.
 
 ## Overview
+
 This code was generated by the [OpenAPI Generator](https://openapi-generator.tech) project.
-By using the [OpenAPI-Spec](https://openapis.org), you can easily generate an API stub.
-This is an example of building API stub interfaces in Java using the Spring framework.
+When generating Spring HTTP Interface clients, you can specify a `clientRegistrationId` parameter to automatically add the `@ClientRegistrationId` annotation to all generated API interfaces.
+
+## Configuration
+
+Add the `clientRegistrationId` property to your generator configuration:
+
+```yaml
+generatorName: spring
+library: spring-http-interface
+additionalProperties:
+  clientRegistrationId: "petstore-oauth"
+```
+
+Or via command line:
+
+```bash
+openapi-generator-cli generate \
+  -g spring \
+  --library spring-http-interface \
+  --additional-properties clientRegistrationId=petstore-oauth \
+  -i petstore.yaml \
+  -o ./output
+```
+
+## Generated Code
+
+The generated interface will include the `@ClientRegistrationId` annotation at the class level:
+
+```java
+@ClientRegistrationId("petstore-oauth")
+public interface PetApi {
+
+    @HttpExchange(
+        method = "GET",
+        value = "/pet/{petId}",
+        accept = { "application/json" }
+    )
+    ResponseEntity<PetDto> getPetById(@PathVariable("petId") Long petId);
+}
+```
+
+## Spring Security Integration
+
+This annotation is part of Spring Security's OAuth2 integration for HTTP Service Clients. It automatically associates OAuth2 tokens with HTTP requests.
+
+### Requirements
+
+- Spring Boot 3.5+
+- Spring Security 6.5+
+
+### Application Properties
+
+Configure your Spring application with the OAuth2 client registration:
+
+```yaml
+spring:
+  security:
+    oauth2:
+      client:
+        registration:
+          petstore-oauth:
+            client-id: your-client-id
+            client-secret: your-client-secret
+            authorization-grant-type: client_credentials
+            scope: read,write
+        provider:
+          petstore-oauth:
+            token-uri: https://auth.example.com/oauth/token
+```
+
+### Bean Configuration
+
+Configure your HTTP Interface beans with OAuth2 support using `RestClient` and the `OAuth2ClientHttpRequestInterceptor`:
 
-The stubs generated can be used in your existing Spring application for HTTP integration with other REST services
-To use auto-generated interfaces you have to create your own configuration which extends default abstract configurator & provide `WebClient` instance via constructor
 ```java
 @Configuration
-public class MyConfiguration extends org.openapitools.configuration.HttpInterfacesAbstractConfigurator {
+public class HttpInterfaceConfig {
+
+    @Bean
+    public PetApi petApi(RestClient.Builder builder, OAuth2AuthorizedClientManager authorizedClientManager) {
+        OAuth2ClientHttpRequestInterceptor interceptor =
+            new OAuth2ClientHttpRequestInterceptor(authorizedClientManager);
+
+        RestClient restClient = builder
+            .baseUrl("https://petstore.example.com/v2")
+            .requestInterceptor(interceptor)
+            .build();
 
-    public MyConfiguration(WebClient myWebClient) { // separately created WebClient instance
-        super(myWebClient);
+        HttpServiceProxyFactory factory = HttpServiceProxyFactory
+            .builderFor(RestClientAdapter.create(restClient))
+            .build();
+
+        return factory.createClient(PetApi.class);
     }
 }
-```
+```
+
+## References
+
+- [Spring Security HTTP Service Client Integration](https://docs.spring.io/spring-security/reference/servlet/oauth2/client/http-service-client.html)
+- [Spring Framework HTTP Interface](https://docs.spring.io/spring-framework/reference/integration/rest-clients.html#rest-http-interface)