simple instructions for creating the EPUB file on OS X

[uroborus]

When I cloned this project to my machine I wasn’t sure how to create
the ebook file, so I emailed Dave Cross and he was kind enough to look
at my terminal output and point out that I needed to install Pandoc.
Once that was installed the ebook was quickly made and I could import
it into iBooks.app

[davorg]

Hi Will,

Thanks for this. I'm not going to merge this PR as I would rather not have OS-specific instructions files in the repo.

But you did convince me that some kind of instructions were needed, so I've added BUILD.md which includes your findings along with a lot more information.

Hope that's acceptable to you. Feel free to send PRs to that.

Cheers,

Dave...