Updated README

Author: gitdagray

README.md

@@ -65,4 +65,5 @@
 - 🔗 [Chapter 1 - Introduction & Setup](https://github.com/gitdagray/next-js-course/tree/main/next01)
 - 🔗 [Chapter 2 - Pages & Layout](https://github.com/gitdagray/next-js-course/tree/main/next02)
 - 🔗 [Chapter 3 - Data Fetching, Dynamic Routes & Metadata](https://github.com/gitdagray/next-js-course/tree/main/next03)
+- 🔗 [Chapter 4 - SSG, SSR, & ISR](https://github.com/gitdagray/next-js-course/tree/main/next04)