Add instructions for compiling to ePub

I personally like to read ebooks with re-flowing text (so I can make the font size bigger). This pull request adds instructions for how to compile as an ePub, since doing so is trivially easy.

There are some other tweaks to the README that could "prettify" it, but I tried not to get too carried away...

(Obviously, feel free to modify this pull request to your liking.)