Provide System.Composition.AttributedModel package readme (dotnet#106781)

* Provide System.Composition.AttributedModel package readme

* Improve after feedback

* Set correct PackageDescription

Author: eNeRGy164

src/libraries/System.Composition.AttributedModel/src/PACKAGE.md

@@ -0,0 +1,140 @@
+## About
+
+<!-- A description of the package and where one can find more documentation -->
+
+`System.Composition.AttributedModel` is part of the Managed Extensibility Framework (MEF) 2.0, a composition library for .NET that enables dependency injection through attributes or conventions.
+
+This package provides the foundational attributes that allow you to declare parts for composition, such as imports, exports, and metadata.
+It is used to mark classes, properties, methods, and constructors for MEF's discovery and composition process.
+
+## Key Features
+
+<!-- The key features of this package -->
+
+* Provides attributes for declaring composable parts, importing dependencies, metadata, and creating shared or non-shared parts
+
+## How to Use
+
+<!-- A compelling example on how to use this package with code, as well as any specific guidelines for when to use the package -->
+
+Mark classes for export and import using attributes.
+
+```csharp
+using System.Composition;
+using System.Composition.Hosting;
+
+var configuration = new ContainerConfiguration()
+    .WithPart<Service>()
+    .WithPart<Application>();
+
+using CompositionHost container = configuration.CreateContainer();
+
+Application app = container.GetExport<Application>();
+app.Service.Execute();
+// Service is running!
+
+[Export]
+public class Application
+{
+    [Import]
+    public Service Service { get; set; }
+}
+
+[Export]
+public class Service
+{
+    public void Execute() => Console.WriteLine("Service is running!");
+}
+```
+
+Metadata can be used to differentiate between multiple exports of the same contract.
+
+```csharp
+using System.Composition;
+using System.Composition.Hosting;
+
+ContainerConfiguration configuration = new ContainerConfiguration()
+    .WithPart<FileLogger>()
+    .WithPart<ConsoleLogger>()
+    .WithPart<Application>();
+
+using CompositionHost container = configuration.CreateContainer();
+
+Application app = container.GetExport<Application>();
+app.Run();
+// Using FileLogger to log.
+// FileLogger: Hello, World!
+// Using ConsoleLogger to log.
+// ConsoleLogger: Hello, World!
+
+public interface ILogger
+{
+    void Log(string message);
+}
+
+[Export(typeof(ILogger))]
+[ExportMetadata("Name", "FileLogger")]
+public class FileLogger : ILogger
+{
+    public void Log(string message) => Console.WriteLine($"FileLogger: {message}");
+}
+
+[Export(typeof(ILogger))]
+[ExportMetadata("Name", "ConsoleLogger")]
+public class ConsoleLogger : ILogger
+{
+    public void Log(string message) => Console.WriteLine($"ConsoleLogger: {message}");
+}
+
+[Export]
+public class Application
+{
+    [ImportMany]
+    public required IEnumerable<Lazy<ILogger, IDictionary<string, object>>> Loggers { get; set; }
+
+    public void Run()
+    {
+        foreach (var logger in Loggers)
+        {
+            var name = logger.Metadata["Name"];
+
+            Console.WriteLine($"Using {name} to log.");
+            logger.Value.Log("Hello, World!");
+        }
+    }
+}
+```
+
+## Main Types
+
+<!-- The main types provided in this library -->
+
+The main types provided by this library are:
+
+* `System.Composition.ExportAttribute`
+* `System.Composition.ImportAttribute`
+* `System.Composition.ExportMetadataAttribute`
+
+## Additional Documentation
+
+<!-- Links to further documentation. Remove conceptual documentation if not available for the library. -->
+
+* [API documentation](https://learn.microsoft.com/dotnet/api/system.composition)
+* [Managed Extensibility Framework (MEF)](https://learn.microsoft.com/dotnet/framework/mef/)
+
+## Related Packages
+
+<!-- The related packages associated with this package -->
+
+* [System.Composition](https://www.nuget.org/packages/System.Composition)
+* [System.Composition.Convention](https://www.nuget.org/packages/System.Composition.Convention)
+* [System.Composition.Hosting](https://www.nuget.org/packages/System.Composition.Hosting)
+* [System.Composition.Runtime](https://www.nuget.org/packages/System.Composition.Runtime)
+* [System.Composition.TypedParts](https://www.nuget.org/packages/System.Composition.TypedParts)
+
+## Feedback & Contributing
+
+<!-- How to provide feedback on this package and contribute to it -->
+
+System.Composition.AttributedModel 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/runtime).

src/libraries/System.Composition.AttributedModel/src/System.Composition.AttributedModel.csproj

@@ -5,17 +5,10 @@
     <StrongNameKeyId>Microsoft</StrongNameKeyId>
     <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile>
     <IsPackable>true</IsPackable>
-    <PackageDescription>Provides common attributes used by System.Composition types.
-
-Commonly Used Types:
-System.Composition.ExportAttribute
-System.Composition.ImportAttribute
-System.Composition.Convention.AttributedModelProvider</PackageDescription>
+    <PackageDescription>Provides the foundational attributes that allow you to declare parts for composition, such as imports, exports, and metadata with the Managed Extensibility Framework (MEF).</PackageDescription>
     <!-- TODO https://github.com/dotnet/runtime/issues/90400: Annotate for nullable reference types -->
     <Nullable>disable</Nullable>
     <NoWarn>$(NoWarn);nullable</NoWarn>
-    <!-- TODO: Add package README file: https://github.com/dotnet/runtime/issues/99358 -->
-    <EnableDefaultPackageReadmeFile>false</EnableDefaultPackageReadmeFile>
   </PropertyGroup>
 
   <ItemGroup>