Provide Microsoft.AspNetCore.TestHost package README (#57790)

Author: MackinnonBuck

src/Hosting/TestHost/src/PACKAGE.md

@@ -0,0 +1,66 @@
+## About
+
+`Microsoft.AspNetCore.TestHost` provides an ASP.NET Core web server for testing middleware in isolation.
+
+## Key Features
+
+* Instantiate an app pipeline containing only the components that you need to test
+* Send custom requests to verify middleware behavior
+
+## How to Use
+
+To use `Microsoft.AspNetCore.TestHost`, follow these steps:
+
+### Installation
+
+```shell
+dotnet add package Microsoft.AspNetCore.TestHost
+```
+
+### Usage
+
+To set up the `TestServer`, configure it in your test project. Here's an example:
+
+```csharp
+[Fact]
+public async Task MiddlewareTest_ReturnsNotFoundForRequest()
+{
+    // Build and start a host that uses TestServer
+    using var host = await new HostBuilder()
+        .ConfigureWebHost(builder =>
+        {
+            builder.UseTestServer()
+                .ConfigureServices(services =>
+                {
+                    // Add any required services that the middleware uses
+                    services.AddMyServices();
+                })
+                .Configure(app =>
+                {
+                    // Configure the processing pipeline to use the middleware
+                    // for the test
+                    app.UseMiddleware<MyMiddleware>();
+                });
+        })
+        .StartAsync();
+
+    var response = await host.GetTestClient().GetAsync("/");
+
+    Assert.Equal(HttpStatusCode.NotFound, response.StatusCode);
+}
+```
+
+## Main Types
+
+The main types provided by this package are:
+
+* `TestServer`: An `IServer` implementation for executing tests
+* `TestServerOptions`: Provides options for configuring a `TestServer`
+
+## Additional Documentation
+
+For additional documentation and examples, refer to the [official documentation](https://learn.microsoft.com/aspnet/core/test/middleware) for testing middleware in ASP.NET Core.
+
+## Feedback &amp; Contributing
+
+`Microsoft.AspNetCore.TestHost` 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).