You can contribute to this book in several ways:
File an issue/start a discussion (just next to the paragraph) where you find an obvious mistake such as a typo/technical error.
If you would like to discuss anything regarding the text matter, use the disqus comments at the bottom of the page.
Pull requests for fixing issues/adding additional content are welcome. However, if you're planning to add considerable content, file an issue to discuss it with me first.
You can also support this book by donating on Gratipay.