install freezes if semantic is not already installed

## 🎉 Description
npm install decentraland-ui
freezes if I have not already run
npm install semantic-ui-react semantic-ui-css

At least a comment to explain/detail this as a necessary step might be useful

Reproduce the problem with `npm install decentraland-ui` in a clean environment, without first installing the Semantic UI packages. Inspect the package dependency declarations and determine whether the install should complete without that manual prerequisite or clearly document it; done means the installation behavior or prerequisite guidance is unambiguous.

Written by the indexing model from the issue text.