[Docs] added the description of the reading order of the config.h files. (#8545)

* added the description of the reading order of the config.h files.

* Update docs/hardware_keyboard_guidelines.md

* Update docs/hardware_keyboard_guidelines.md

* Added a description of post_config.h.

* sample bug fix

* sample update

* Update docs/hardware_keyboard_guidelines.md

* Update docs/hardware_keyboard_guidelines.md

* update docs/hardware_keyboard_guidelines.md

* Update docs/hardware_keyboard_guidelines.md

docs/hardware_keyboard_guidelines.md

@@ -61,6 +61,57 @@ This file is used by the [QMK API](https://github.com/qmk/qmk_api). It contains
 
 All projects need to have a `config.h` file that sets things like the matrix size, product name, USB VID/PID, description and other settings. In general, use this file to set essential information and defaults for your keyboard that will always work.
 
+The `config.h` files can also be placed in sub-folders, and the order in which they are read is as follows:
+
+* `keyboards/top_folder/config.h`
+  * `keyboards/top_folder/sub_1/config.h`
+    * `keyboards/top_folder/sub_1/sub_2/config.h`
+      * `keyboards/top_folder/sub_1/sub_2/sub_3/config.h`
+        * `keyboards/top_folder/sub_1/sub_2/sub_3/sub_4/config.h`
+          * `users/a_user_folder/config.h`
+          * `keyboards/top_folder/keymaps/a_keymap/config.h`
+        * `keyboards/top_folder/sub_1/sub_2/sub_3/sub_4/post_config.h`
+      * `keyboards/top_folder/sub_1/sub_2/sub_3/post_config.h`
+    * `keyboards/top_folder/sub_1/sub_2/post_config.h`
+  * `keyboards/top_folder/sub_1/post_config.h`
+* `keyboards/top_folder/post_config.h`
+
+The `post_config.h` file can be used for additional post-processing, depending on what is specified in the `config.h` file. For example, if you define the `IOS_DEVICE_ENABLE` macro in your keymap-level `config.h` file as follows, you can configure more detailed settings accordingly in the `post_config.h` file:
+
+* `keyboards/top_folder/keymaps/a_keymap/config.h`
+  ```c
+  #define IOS_DEVICE_ENABLE
+  ```
+* `keyboards/top_folder/post_config.h`
+  ```c
+  #ifndef IOS_DEVICE_ENABLE
+    // USB_MAX_POWER_CONSUMPTION value for this keyboard
+    #define USB_MAX_POWER_CONSUMPTION 400
+  #else
+    // fix iPhone and iPad power adapter issue
+    // iOS device need lessthan 100
+    #define USB_MAX_POWER_CONSUMPTION 100
+  #endif
+  
+  #ifdef RGBLIGHT_ENABLE
+    #ifndef IOS_DEVICE_ENABLE
+      #define RGBLIGHT_LIMIT_VAL 200
+      #define RGBLIGHT_VAL_STEP 17
+    #else
+      #define RGBLIGHT_LIMIT_VAL 35
+      #define RGBLIGHT_VAL_STEP 4
+    #endif
+    #ifndef RGBLIGHT_HUE_STEP
+      #define RGBLIGHT_HUE_STEP 10
+    #endif
+    #ifndef RGBLIGHT_SAT_STEP
+      #define RGBLIGHT_SAT_STEP 17
+    #endif
+  #endif
+  ```
+
+?> If you define options using `post_config.h` as in the above example, you should not define the same options in the keyboard- or user-level `config.h`.
+
 ### `rules.mk`
 
 The presence of this file means that the folder is a keyboard target and can be used in `make` commands. This is where you setup the build environment for your keyboard and configure the default set of features.