doc: update info about directories and files

README.md

@@ -11,12 +11,16 @@ with my solutions to the questions in each quiz.
 
 ## Directories and files
 
-* [`docs`](./docs): Markdown documents.
-  * [`notes`](./docs/notes)
-    * `ChapterXX`: My notes about the concepts explained in Chapter XX of the
-      text.
-  * [`quiz`](./docs/quiz)
-    * `chapterXX.md`: My additional quiz for Chapter XX of the text.
+* [`docs`](./docs): Markdown documents including notes and quizzes.
+  * [`en`](./docs/en): Source documents written in English.
+    * [`notes`](./docs/en/notes)
+      * `ChapterXX`: My notes about the concepts explained in Chapter XX of the
+        text.
+    * [`quiz`](./docs/en/quiz)
+      * `chapterXX.md`: My additional quiz for Chapter XX of the text.
+  * [`ko`](./docs/ko): Korean translation of the source documents in the `en`
+    directory.
+
 * [`TPIL`](./TPIL): My solutions to the exercises and questions.
   * `ChapterXX`: Chapter XX of the text.
     * `Question*`: Solutions to the question(s) of my quiz. Yet to be public.