Update README to mention yarn?

[ducks]

https://yarnpkg.com was just released and seems pretty cool.

Too early to update README to mention/use yarn?

They add a lockfile that may be worth versioning
https://yarnpkg.com/en/docs/yarn-lock#toc-check-into-source-control

[alexcrichton]

Looks like this is being done in #457

[ducks]

Dang. It is indeed.