Update disclaimer on age of book and maintenance

[iteles]

I've seen a couple of people refer to this repo this year (2020) and I reckon it's time we add a disclaimer here to note:

• This book was published in 2008
• JS has moved on and so have we.
• We are no longer maintaining this repo

There is already a disclaimer currently but it needs a little adjusting.

https://twitter.com/YvanDaSilva/status/1258042668828983297

[nelsonic]

When people use Twitter like it's SnapChat (disappearing messages 🙄 ...)

P.S. @iteles would you mind if I create a PR to add the disclaimer? 💭

[nelsonic]

I want to reference this tutorial as a prerequisite for a tutorial I'm busy writing so I would like the intro to be tidy ...
Right now the intro links to #33 which is about transferring the repo to @dwyl ...

I think it's meant to refer to this issue. Either way it could do with some TLC.