learnings from an #npm > #yarn migration:

- yarn's official docs are v sparse. better technical details here: https://code.facebook.com/posts/1840075619545360
- deterministic installs + committable lockfiles = 💞
- by design, yarn doesn't create binary symlinks for transitive deps (https://github.com/yarnpkg/yarn/issues/2874)
- relying on transitive binaries is hacky anyway; better to install explicitly
- ⚠ it's NOT a drop-in replacement.