We have now released 5.7.0!
As Mikko said, we have added lots of changes, but there are not that much user-facing changes (yet). E.g. the first React components are only visible for marketplace admins. However, there are quite many changes to build environment. To build React components we use Webpack (node environment) and the build is started with Foreman. You can read more from our CHANGELOG & UPGRADE steps as well as from client/README.md