Broken link in README.md

[visortelle]

I got 404 after clicking the link:

[kotx]

Seems the gh-pages branch is missing a current folder.
The link for the book index in here is broken as well:

The book is also available for the master branch and older versions published after 0.11.1. See the book index.

[insprac]

I'm looking at the repo for the first time and the book link is broken like kotx said, which is less than ideal since it's the first link in "Getting Started"