Add Readme for Framework (#27918)

src/Framework/README.md

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+# Framework
+
+Contains packages that bundle reference and implementation assembles for Microsoft.AspNetCore.App.Runtime shared framework and for use in targeting pack installers.
+
+## Description
+
+The following contains a description of each sub-directory in the `Framework` directory.
+
+- `App.Ref.Internal`: Empty project for identifying the non-stable version of AspNetCore at the time the targeting pack was last built. This package does not ship and is for internal use.
+- `App.Ref`: Contains reference assemblies, documentation, and other design-time assets
+- `App.Runtime`: Provides a default set of APIs for building ASP.NET Core applications, and assets used for self-contained deployments.
+
+## Development Setup
+
+### Build
+
+To build this specific project from source,  follow the instructions [on building a subset of the code](../../docs/BuildFromSource.md#building-a-subset-of-the-code).
+
+### Test
+
+To run the tests for this project, [run the tests on the command line](../../docs/BuildFromSource.md#running-tests-on-command-line) in this directory.
+
+## More Information
+
+For more information, see the [ASP.NET Core README](../../README.md).