Provide System.Composition package readme (dotnet#106443)

* Provide System.Composition package readme

* Fix empty lines

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

@@ -0,0 +1,77 @@
+## About
+
+<!-- A description of the package and where one can find more documentation -->
+
+Provides the Managed Extensibility Framework (MEF) 2.0, a lightweight, attribute-driven Dependency Injection (DI) container.
+
+MEF simplifies the composition of applications by allowing components to be loosely coupled and dynamically discovered.
+This package supports the development of modular and maintainable applications by enabling parts to be composed at runtime.
+
+## Key Features
+
+<!-- The key features of this package -->
+
+* Components are discovered and composed using attributes.
+* Provides dependency injection capabilities for loosely coupled modules.
+
+## 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 -->
+
+Running code from a discovered component.
+
+```csharp
+using System.Composition;
+using System.Composition.Hosting;
+
+var configuration = new ContainerConfiguration().WithPart<Service>();
+
+using var container = configuration.CreateContainer();
+
+var service = container.GetExport<Service>();
+service.Execute();
+// Output: Service is running!
+
+[Export]
+public class Service
+{
+    public void Execute() => Console.WriteLine("Service is running!");
+}
+```
+
+## 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.Convention.ConventionBuilder`
+* `System.Composition.Hosting.CompositionHost`
+* `System.Composition.CompositionContext`
+* `System.Composition.CompositionContextExtensions`
+
+## 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.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.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 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/src/System.Composition.csproj

@@ -8,21 +8,10 @@
     <NoWarn>$(NoWarn);NU5128</NoWarn>
     <!-- Reference dependencies to persiste them in the project's project.assets.json file which is used to calculate package dependencies. -->
     <NoTargetsDoNotReferenceOutputAssemblies>false</NoTargetsDoNotReferenceOutputAssemblies>
-    <PackageDescription>This package provides a version of the Managed Extensibility Framework (MEF) that is lightweight and specifically optimized for high throughput scenarios, such as the web.
-
-Commonly Used Types:
-System.Composition.ExportAttribute
-System.Composition.ImportAttribute
-System.Composition.Convention.ConventionBuilder
-System.Composition.Hosting.CompositionHost
-System.Composition.CompositionContext
-System.Composition.CompositionContextExtensions</PackageDescription>
-
+    <PackageDescription>Provides a version of the Managed Extensibility Framework (MEF) that is lightweight and specifically optimized for high throughput scenarios, such as the web.</PackageDescription>
     <!-- TODO https://github.com/dotnet/runtime/issues/90400: Annotate for nullable reference types -->
     <Nullable>disable</Nullable>
     <NoWarn>$(NoWarn);nullable</NoWarn>
-    <!-- TODO: Add package README file: https://github.com/dotnet/runtime/issues/99358 -->
-    <EnableDefaultPackageReadmeFile>false</EnableDefaultPackageReadmeFile>
   </PropertyGroup>
 
   <ItemGroup>