Justify this directory's existence

include/grpcpp/impl/codegen/README.md

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+# Welcome to `impl/codegen`
+
+## Why is this directory here?
+
+This directory exists so that generated code can include selected files upon
+which it depends without having to depend on the entire gRPC C++ library. This
+is particularly relevant for users of bazel, particularly if they use the
+multi-lingual `proto_library` target type. Generated code that uses this target
+only depends on the gRPC C++ targets associated with these header files, not the
+entire gRPC C++ codebase since that would make the build time of these types of
+targets excessively large (particularly when they are not even C++ specific).
+
+## What should user code do?
+
+User code should *not* include anything from this directory. Only generated code
+and gRPC library code should include contents from this directory. User code
+should instead include contents from the main `grpcpp` directory or its
+accessible subcomponents like `grpcpp/support`.