Provide Microsoft.AspNetCore.JsonPatch package README (#57792)

Author: MackinnonBuck

src/Features/JsonPatch/src/PACKAGE.md

@@ -0,0 +1,100 @@
+## About
+
+`Microsoft.AspNetCore.JsonPatch` provides ASP.NET Core support for JSON PATCH requests.
+
+## How to Use
+
+To use `Microsoft.AspNetCore.JsonPatch`, follow these steps:
+
+### Installation
+
+```shell
+dotnet add package Microsoft.AspNetCore.JsonPatch
+dotnet add package Microsoft.AspNetCore.Mvc.NewtonsoftJson
+```
+
+### Configuration
+
+To enable JSON Patch support, call `AddNewtonsoftJson` in your ASP.NET Core app's `Program.cs`:
+```csharp
+var builder = WebApplication.CreateBuilder(args);
+
+builder.Services.AddControllers()
+    .AddNewtonsoftJson();
+```
+
+#### Configure when using `System.Text.Json`
+
+To add support for JSON Patch using `Newtonsoft.Json` while continuing to use `System.Text.Json` for other input and output formatters:
+
+1. Update your `Program.cs` with logic to construct a `NewtonsoftJsonPatchInputFormatter`:
+    ```csharp
+    static NewtonsoftJsonPatchInputFormatter GetJsonPatchInputFormatter()
+    {
+        var builder = new ServiceCollection()
+            .AddLogging()
+            .AddMvc()
+            .AddNewtonsoftJson()
+            .Services.BuildServiceProvider();
+
+        return builder
+            .GetRequiredService<IOptions<MvcOptions>>()
+            .Value
+            .InputFormatters
+            .OfType<NewtonsoftJsonPatchInputFormatter>()
+            .First();
+    }
+    ```
+2. Configure the input formatter:
+    ```csharp
+    var builder = WebApplication.CreateBuilder(args);
+
+    builder.Services.AddControllers(options =>
+    {
+        options.InputFormatters.Insert(0, GetJsonPatchInputFormatter());
+    });
+    ```
+
+### Usage
+
+To define an action method for a JSON Patch in an API controller:
+1. Annotate it with the `HttpPatch` attribute
+2. Accept a `JsonPatchDocument<TModel>`
+3. Call `ApplyTo` on the patch document to apply changes
+
+For example:
+
+```csharp
+[HttpPatch]
+public IActionResult JsonPatchWithModelState(
+    [FromBody] JsonPatchDocument<Customer> patchDoc)
+{
+    if (patchDoc is not null)
+    {
+        var customer = CreateCustomer();
+
+        patchDoc.ApplyTo(customer, ModelState);
+
+        if (!ModelState.IsValid)
+        {
+            return BadRequest(ModelState);
+        }
+
+        return new ObjectResult(customer);
+    }
+    else
+    {
+        return BadRequest(ModelState);
+    }
+}
+```
+
+In a real app, the code would retrieve the data from a store such as a database and update the database after applying the patch.
+
+## Additional Documentation
+
+For additional documentation and examples, refer to the [official documentation](https://learn.microsoft.com/aspnet/core/web-api/jsonpatch) on JSON Patch in ASP.NET Core.
+
+## Feedback &amp; Contributing
+
+`Microsoft.AspNetCore.JsonPatch` 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).