Add DifferentiableProgrammingImplementation.md to docs/README.md (#41123)

* Update README.md

* Update docs/README.md

Co-authored-by: Robert Widmann <[email protected]>

Co-authored-by: Robert Widmann <[email protected]>

Author: philipturner

docs/README.md

@@ -102,6 +102,8 @@ documentation, please create a thread on the Swift forums under the
 - [DependencyAnalysis.md](/docs/DependencyAnalysis.md):
   Describes different kinds of dependencies across files in the same module,
   important for understanding incremental builds.
+- [DifferentiableProgrammingImplementation.md](/docs/DifferentiableProgrammingImplementation.md):
+  Describes how automatic differentiation is implemented in the Swift compiler.
 - C and ObjC interoperability: Clang Importer and PrintAsClang
   - [CToSwiftNameTranslation.md](/docs/CToSwiftNameTranslation.md):
     Describes how C and ObjC entities are imported into Swift