Provide Microsoft.AspNetCore.Mvc.Testing package README (#57736)

Co-authored-by: Martin Costello <martin@martincostello.com>

Author: MackinnonBuck

src/Mvc/Mvc.Testing/src/PACKAGE.md

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+## About
+
+`Microsoft.AspNetCore.Mvc.Testing` provides support for writing integration tests for ASP.NET Core apps that utilize MVC or Minimal APIs.
+
+## Key Features
+
+* Copies the dependencies file (`.deps.json`) from the System Under Test (SUT) into the test project's `bin` directory
+* Sets the [content root](https://learn.microsoft.com/aspnet/core/fundamentals/#content-root) to the SUT's project root so that static files are found during test execution
+* Provides the [`WebApplicationFactory`](https://learn.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.testing.webapplicationfactory-1) class to streamline bootstrapping the SUT with [`TestServer`](https://learn.microsoft.com/dotnet/api/microsoft.aspnetcore.testhost.testserver)
+
+## How to Use
+
+To use `Microsoft.AspNetCore.Mvc.Testing`, follow these steps:
+
+### Installation
+
+To install the package, run the following command from the directory containing the test project file:
+
+```shell
+dotnet add package Microsoft.AspNetCore.Mvc.Testing
+```
+
+### Configuration
+
+To configure the test app, follow these steps:
+
+1. Specify the Web SDK in the test project file (`<Project Sdk="Microsoft.NET.Sdk.Web">`).
+2. Add references to the following packages:
+    * `xunit`
+    * `xunit.runner.visualstudio`
+    * `Microsoft.NET.Test.Sdk`
+3. Add a test class to the test project:
+    ```csharp
+    public class BasicTests
+        : IClassFixture<WebApplicationFactory<Program>>
+    {
+        private readonly WebApplicationFactory<Program> _factory;
+
+        public BasicTests(WebApplicationFactory<Program> factory)
+        {
+            _factory = factory;
+        }
+
+        [Theory]
+        [InlineData("/")]
+        [InlineData("/Index")]
+        [InlineData("/About")]
+        [InlineData("/Privacy")]
+        [InlineData("/Contact")]
+        public async Task Get_EndpointsReturnSuccessAndCorrectContentType(string url)
+        {
+            // Arrange
+            var client = _factory.CreateClient();
+
+            // Act
+            var response = await client.GetAsync(url);
+
+            // Assert
+            response.EnsureSuccessStatusCode(); // Status Code 200-299
+            Assert.Equal("text/html; charset=utf-8",
+                response.Content.Headers.ContentType.ToString());
+        }
+    }
+    ```
+
+## Additional Documentation
+
+For additional documentation and examples, refer to the [official documentation](https://learn.microsoft.com/aspnet/core/test/integration-tests) on integration testing in ASP.NET Core.
+
+## Feedback &amp; Contributing
+
+`Microsoft.AspNetCore.Mvc.Testing` 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).