Document which Babel transforms are needed

[stubailo]

Using ES2015 is great, but the tutorial uses some features such as static class properties which are still in the proposal stage. In this case, I think it's important to document which features are being used outside of ES2015, and tell people how to get those Babel transforms.

I heard about this in the context of Meteor, which is relatively conservative in that it supports mostly things that have made it into an ECMAScript specification, so doesn't include static class properties.

In this case the transform is: https://babeljs.io/docs/plugins/transform-class-properties/

But there might be others.