Provide Microsoft.AspNetCore.Authentication.OpenIdConnect package README (#57731)

Author: MackinnonBuck

src/Security/Authentication/OpenIdConnect/src/PACKAGE.md

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+## About
+
+`Microsoft.AspNetCore.Authentication.OpenIdConnect` provides middleware that enables an application to support the OpenID Connect authentication workflow.
+
+## Key Features
+
+* Single sign-on and single sign-out support
+* Integration with external identity providers
+* Token validation and management
+* Configuration and mapping of user claims
+
+## How to Use
+
+To use `Microsoft.AspNetCore.Authentication.OpenIdConnect`, follow these steps:
+
+### Installation
+
+```shell
+dotnet add package Microsoft.AspNetCore.Authentication.OpenIdConnect
+```
+
+### Configuration
+
+To configure `Microsoft.AspNetCore.Authentication.OpenIdConnect`, you need to add the necessary services and middleware to your application.
+
+1. In the `Program.cs` of your ASP.NET Core app, add the following code to register the OpenID Connect authentication services:
+    ```csharp
+    builder.Services
+        .AddAuthentication(options =>
+        {
+            options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
+            options.DefaultChallengeScheme = OpenIdConnectDefaults.AuthenticationScheme;
+        })
+        .AddCookie()
+        .AddOpenIdConnect(options =>
+        {
+            // Configure the authentication options
+            options.SignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
+            options.Authority = "your-identity-provider";
+            options.ClientId = "your-client-id";
+            options.ClientSecret = "your-client-secret-from-user-secrets-or-keyvault";
+            options.ResponseType = "code";
+            options.Scope.Add("profile");
+            options.SaveTokens = true;
+        });
+    ```
+
+    Make sure to replace `your-identity-provider`, `your-client-id`, and `your-client-secret-from-user-secrets-or-keyvault`, with the appropriate values for your application and identity provider.
+
+2. Add the following code to enable the OpenID Connect authentication middleware:
+    ```csharp
+    var app = builder.Build();
+
+    app.UseAuthentication();
+    ```
+    This ensures that the authentication middleware is added to the request pipeline.
+
+## Main Types
+
+The main types provided by `Microsoft.AspNetCore.Authentication.OpenIdConnect` are:
+
+* `OpenIdConnectOptions`: Represents the options for configuring the OpenID Connect authentication middleware
+* `OpenIdConnectEvents`: Provides event handlers for various stages of the OpenID Connect authentication workflow
+
+For more information on these types and their usage, refer to the [official documentation](https://learn.microsoft.com/dotnet/api/microsoft.aspnetcore.authentication.openidconnect).
+
+## Additional Documentation
+
+For additional documentation on using OpenID Connect authentication in ASP.NET Core, you can refer to the following resources:
+
+* [ASP.NET Core Authentication](https://learn.microsoft.com/aspnet/core/security/authentication)
+* [OpenID Connect](https://openid.net/developers/how-connect-works)
+* [Entra ID documentation](https://learn.microsoft.com/entra/identity)
+
+## Feedback & Contributing
+
+`Microsoft.AspNetCore.Authentication.OpenIdConnect` is released as open-source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/aspnetcore).