Add chapter 3 coding along video link (#572)

Author: rasbt

ch03/README.md

@@ -9,4 +9,13 @@
 ## Bonus Materials
 
 - [02_bonus_efficient-multihead-attention](02_bonus_efficient-multihead-attention) implements and compares different implementation variants of multihead-attention
-- [03_understanding-buffers](03_understanding-buffers) explains the idea behind PyTorch buffers, which are used to implement the causal attention mechanism in chapter 3
+- [03_understanding-buffers](03_understanding-buffers) explains the idea behind PyTorch buffers, which are used to implement the causal attention mechanism in chapter 3
+
+
+
+In the video below, I provide a code-along session that covers some of the chapter contents as supplementary material.
+
+<br>
+<br>
+
+[![Link to the video](https://img.youtube.com/vi/-Ll8DtpNtvk/0.jpg)](https://www.youtube.com/watch?v=Ll8DtpNtvk)