Provide System.Composition.Runtime package readme (#106784)

* Provide System.Composition.Runtime package readme

* Improve after feedback

Author: eNeRGy164

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

@@ -0,0 +1,87 @@
+## About
+
+<!-- A description of the package and where one can find more documentation -->
+
+`System.Composition.Runtime` 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 enables the discovery and composition of parts in applications using MEF 2.0.
+It offers the runtime implementation needed for managing composable parts, resolving dependencies, and dynamically wiring components together.
+
+## Key Features
+
+<!-- The key features of this package -->
+
+* Facilitates runtime discovery and composition of 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 -->
+
+Resolve dependencies on the fly and can be useful for dynamically loaded components or plugins.
+
+```csharp
+using System.Composition;
+using System.Composition.Hosting;
+
+var configuration = new ContainerConfiguration()
+    .WithPart<Service>();
+
+using CompositionHost container = configuration.CreateContainer();
+ 
+var consumer = new Consumer(container);
+consumer.Execute();
+// Service is running.
+
+public interface IService
+{
+    void Run();
+}
+
+[Export(typeof(IService))]
+public class Service : IService
+{
+    public void Run() => Console.WriteLine("Service is running.");
+}
+
+public class Consumer(CompositionContext context)
+{
+    public void Execute()
+    {
+        // Use the context to resolve the service
+        var service = context.GetExport<IService>();
+        service.Run();
+    }
+}
+```
+
+## Main Types
+
+<!-- The main types provided in this library -->
+
+The main type provided by this library is:
+
+* `System.Composition.CompositionContext`
+
+## 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.compositioncontext)
+* [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.AttributedModel](https://www.nuget.org/packages/System.Composition.AttributedModel)
+* [System.Composition.Convention](https://www.nuget.org/packages/System.Composition.Convention)
+* [System.Composition.Hosting](https://www.nuget.org/packages/System.Composition.Hosting)
+* [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.Runtime 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.Runtime/src/System.Composition.Runtime.csproj

@@ -7,15 +7,10 @@
     <StrongNameKeyId>Microsoft</StrongNameKeyId>
     <UseCompilerGeneratedDocXmlFile>false</UseCompilerGeneratedDocXmlFile>
     <IsPackable>true</IsPackable>
-    <PackageDescription>Contains runtime components of the Managed Extensibility Framework.
-
-Commonly Used Types:
-System.Composition.CompositionContext</PackageDescription>
+    <PackageDescription>Contains runtime components of 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>