Added in Next.js (#6186)

* Added in Next.js

* Alphabetical ordering

* Incorrect lines btw

* correct title
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NEric Hellman <eric@hellman.net>
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -1270,6 +1270,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 #### Next.js
 
 * [Mastering Next.js](https://masteringnextjs.com)
+* [Next.js Crash Course 2021](https://www.youtube.com/watch?v=mTz0GXj8NN0) - Brad Traversy (Traversy Media)
 * [The Next.js Handbook](https://flaviocopes.com/page/nextjs-handbook/) - Flavio Copes (HTML, PDF) *(email required for PDF)*