add link to supplementary ch02 video (#553)

Author: rasbt

ch02/README.md

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 - [04_bonus_dataloader-intuition](04_bonus_dataloader-intuition) contains optional (bonus) code to explain the data loader more intuitively with simple numbers rather than text.
 
 - [05_bpe-from-scratch](05_bpe-from-scratch) contains (bonus) code that implements and trains a GPT-2 BPE tokenizer from scratch.
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+In the video below, I provide a code-along session that covers some of the chapter contents as supplementary material.
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+[![Link to the video](https://img.youtube.com/vi/341Rb8fJxY0/0.jpg)](https://www.youtube.com/watch?v=yAcWnfsZhzo)