resolve comments - add info about folder structure in README.md

Author: fs-eire

include/onnxruntime/ep/README.md

@@ -2,6 +2,18 @@
 
 This folder contains a set of C++ header files. They are used specifically for allowing ONNX Runtime internal kernel-based EPs to use the plugin-style EP API while keep minimal changes to existing code.
 
+### Folder Structure
+
+There are 2 types of header files:
+
+- General header files for plugin EP. This may include utilities, macros and shared routines that depending on ONNX Runtime public API only. There are multiple places for header files of this category (which we are going to unify them to one place. There is an ongoing discussion about unifying shared headers for plugin EPs):
+  - `include/onnxruntime/ep/` (#26919)
+  - `onnxruntime/test/autoep/library/plugin_ep_utils.h`
+  - `include/onnxruntime/core/providers/utils/` (#25753)
+
+- Header files specifically used for supporting WebGPU EP and CUDA EP to use EP APIs. These header files do not only depend on ONNX Runtime public API, but also depend on ONNX Runtime internal headers.
+  - `include/onnxruntime/ep/adapter/`
+
 ### Usage
 
 Make sure to include "ep/_pch.h" for all source code in the implementation. Using PCH compiler flag is recommended.