docs(readme): add notice of not fully working in chromium

This is because CSS variables are used, which are currently (at time of
writing) only implemented in firefox.

The feature is put [on hold] [1] for chromium.

[1]: https://groups.google.com/a/chromium.org/d/msg/blink-dev/ScKw9zYRkBc/W5_XaS7ZzzsJ