Provide Microsoft.AspNetCore.Authentication.Certificate package README (#57811)

Author: MackinnonBuck

src/Security/Authentication/Certificate/src/PACKAGE.md

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+## About
+
+`Microsoft.AspNetCore.Authentication.Certificate` provides ASP.NET Core middleware that enables an application to support certificate authentication.
+
+## How to Use
+
+To use `Microsoft.AspNetCore.Authentication.Certificate`, follow these steps:
+
+### Installation
+
+```shell
+dotnet add package Microsoft.AspNetCore.Authentication.Certificate
+```
+
+### Configuration
+
+1. Acquire an HTTPS certificate, install it, and [configure your server](https://learn.microsoft.com/aspnet/core/security/authentication/certauth#configure-your-server-to-require-certificates) to require certificates
+2. Configure the middleware in your APS.NET Core app's `Program.cs`:
+    ```csharp
+    var builder = WebApplication.CreateBuilder(args);
+
+    builder.Services.AddAuthentication(
+            CertificateAuthenticationDefaults.AuthenticationScheme)
+        .AddCertificate();
+
+    var app = builder.Build();
+
+    app.UseAuthentication();
+    ```
+
+## Main Types
+
+* `CertificateAuthenticationOptions`: Provides options to configure certificate authentication
+* `ICertificateValidationCache`: Provides a cache used to store `AuthenticateResult` results after the certificate has been validated
+
+## Additional Documentation
+
+For additional documentation and examples, refer to the [official documentation](https://learn.microsoft.com/aspnet/core/security/authentication/certauth) on certificate authentication in ASP.NET Core.
+
+## Feedback & Contributing
+
+`Microsoft.AspNetCore.Authentication.Certificate` 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).