Rebrand IdentityModel to Duende in README

Updated README to reflect rebranding to Duende.AspNetCore.Authentication.OAuth2Introspection and provided new package reference.

Author: damianh

README.md

@@ -1,35 +1,6 @@
 # IdentityModel.AspNetCore.OAuth2Introspection
 
-ASP.NET Core authentication handler for OAuth 2.0 token introspection
-
-https://tools.ietf.org/html/rfc7662
-
-## Configuration
-
-```csharp
-services.AddAuthentication(OAuth2IntrospectionDefaults.AuthenticationScheme)
-    .AddOAuth2Introspection(options =>
-    {
-        options.Authority = "https://base_address_of_token_service";
-
-        options.ClientId = "client_id_for_introspection_endpoint";
-        options.ClientSecret = "client_secret_for_introspection_endpoint";
-    });
-```
-
-## Configuring Backchannel HTTP Client
-
-If configuration, such as using a proxy, is required for the HTTP client calling the Authority then it can be done by registering a named HTTP Client as follows
-
-```csharp
-services.AddHttpClient(OAuth2IntrospectionDefaults.BackChannelHttpClientName) 
-    .AddHttpMessageHandler(() => 
-    {
-        //Configure client/handler for the back channel HTTP Client here
-        return new HttpClientHandler
-            {
-                UseProxy = true,
-                Proxy = new WebProxy(WebProxyUri, true)
-            };
-    }
-```
+> [!IMPORTANT]  
+> `IdentityModel.AspNetCore.OAuth2Introspection` has been rebranded to `Duende.AspNetCore.Authentication.OAuth2Introspection` and henceforth will be managed 
+> in our [FOSS repository](https://github.com/DuendeSoftware/foss) under the same Apache2 licence.
+>  - Please update your nuget package references to the [new package](https://www.nuget.org/packages/Duende.AspNetCore.Authentication.OAuth2Introspection).