Update README.md

[deveyNull]

I added a gitbook to make the course material more accessible.

[kazamatzuri]

that linked book feels a lot like a link farming site/spammy :/ about every second page pushes a 'sign up for the free course' the rest are links to writeups on other pages?

[deveyNull]

Not link farming, it's the full content of the course but in a git fork-able format that doesn't require messing around with the actual course website. The links to other writeups are selected because they don't suck (in process of rewriting most of them anyway), but there's plenty of other content in there