Adding PACKAGE.md to System.Runtime.Caching package

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

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+## About
+
+[System.Runtime.Caching](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.caching?view=dotnet-plat-ext-7.0)/[System.Runtime.Caching.MemoryCache](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.caching.memorycache?view=dotnet-plat-ext-7.0) ([NuGet package](https://www.nuget.org/packages/System.Runtime.Caching/)) can be used with:
+
+* .NET Standard 2.0 or later.
+* Any [.NET implementation](/dotnet/standard/net-standard#net-implementation-support) that targets .NET Standard 2.0 or later. For example, ASP.NET Core 3.1 or later.
+* .NET Framework 4.5 or later.
+
+[Microsoft.Extensions.Caching.Memory](https://www.nuget.org/packages/Microsoft.Extensions.Caching.Memory/)/`IMemoryCache` (described in this article) is recommended over `System.Runtime.Caching`/`MemoryCache` because it's better integrated into ASP.NET Core. For example, `IMemoryCache` works natively with ASP.NET Core [dependency injection](xref:fundamentals/dependency-injection).
+
+Use `System.Runtime.Caching`/`MemoryCache` as a compatibility bridge when porting code from ASP.NET 4.x to ASP.NET Core.
+
+## Key Features
+
+* MemoryCache feature represents the type that implements an in-memory cache.
+
+## Main Types
+
+The main types provided by this library are:
+
+* `System.Runtime.Caching.MemmoryCache`
+
+## Remarks
+
+[MemoryCache.PhysicalMemeoryLimit](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.caching.memorycache.physicalmemorylimit?view=dotnet-plat-ext-7.0) property is only supported on windows.
+
+## Addtional Documentation
+
+* [Caching in .NET](https://learn.microsoft.com/en-us/dotnet/core/extensions/caching)
+* [Cache in-memory in ASP.NET Core](https://learn.microsoft.com/en-us/aspnet/core/performance/caching/memory?view=aspnetcore-7.0 )
+
+## Related Packages
+
+* [Microsoft.Extensions.Caching.Memory](https://www.nuget.org/packages/Microsoft.Extensions.Caching.Memory/)
+
+## Feedback & Contributing
+
+System.Runtime.Caching 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).