Repo can't make up its mind if we use npm or Yarn

[ExplodingCabbage]

On the one hand, we've got a yarn.lock file (and no npm-shrinkwrap.json or package-lock.json). On the other hand, our CONTRIBUTING.md tells people to use npm:

Building

npm install
npm test

The npm test -- dev implements watching for tests within Node and karma start may be used for manual testing in browsers.

I guess we should update the docs to reflect that the repo is in fact using Yarn.