sound change rules

docs/index.md

@@ -7,6 +7,8 @@ hero:
   tagline: "Korean grammar edited"
 ---
 
+[Sound rules](./ภาษาเกาหลีเรียนง่าย/1/04-sound-rules.md)
+
 - [Lesson 1: Basic Sentences](./howtostudykorean/unit-1/01-sentences.md)
 - [Lesson 2: Particles](./howtostudykorean/unit-1/02-particles.md)
 - [Lesson 3: Verbs/Adjectives](./howtostudykorean/unit-1/03-verbs.md)