Add a book to readings (see description)

[justus-3]

Add "Code: The Hidden Language of Computer Hardware and Software" to extras/readings.md

[waciumawanjohi]

In the future, please add justification for why the resource should be added.

In this case, when I searched the title I found evidence that the book is well reviewed and has been valuable for CS learners.

High user reviews:

• https://www.goodreads.com/book/show/44882.Code
• https://www.amazon.com/Code-Language-Computer-Hardware-Software/dp/0137909101

CS adherents describing the book as useful to learners:

• https://flatironschool.com/blog/book-review-code-the-hidden-language-of-computer-hardware-and-software/
• https://www.reddit.com/r/learnprogramming/comments/11f0chv/is_code_the_hidden_language_of_computer_hardware/