Release download links

[wanderrful]

Feel free to close or delete this after reading:

I recommend migrating the epub itself into a release so that we can go there to download as opposed to the readme.md... or maybe as an alternative instead.

Thank you for making this into an epub!

[kopp]

Is there a way to run the builds automatically (e.g. by travis), to include the latest version of the book in the releases?