Add a note about performance to README

[utkarshkukreti]

Reaml + React and Reaml + Preact were added to js-framework-benchmark in March 2020.

The results are pretty good -- only about 2-3% slower than the pure JS implementation that uses the same library.

I think this should be mentioned in the README for those concerned about the non-zero cost binding of this package vs reason-react.