Make Yakread open-source

[jacobobryant]

https://yakread.com didn't work out as a full-time business, but it's still a useful and interesting app. I'm in the process of rewriting it to use indexes and Pathom, and will likely use it as a test project for other new Biff features too.

Some time after the rewrite is finished, I'll make it open-source and mention it more prominently in the Biff documentation as a flagship/example/demo project.