[janet/en] Adds external ebook link for further reading.

[nathanielkaiserxyz]

• [ ✓] I solemnly swear that this is all original content of which I am the original author
• [ ✓] Pull request title is prepended with [language/lang-code] (example [python/fr] or [java/en])
• [✓ ] Pull request touches only one file (or a set of logically related files with similar changes made)
• [✓ ] Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)

Janet for Mortals is a great book with a built in repl for testing/learning.

[vendethiel]

I’m fine with the guide (the book is free), but we don’t credit such edits I’m afraid.

[nathanielkaiserxyz]

I wasn't sure what to do on that actually I'll remove the credit.

[vendethiel]

Appreciated. Thank you!