Note the need for an Object.assign polyfill

Not sure if this is enough to just place it in the note. Or whether the MDN polyfill link is appropriate, I wanted to provide a solution along with the warning. Another option is pointing them to the react polyfil in the source...

refrences #2488
How about:

> This experimental syntax compiles into an ES6 `Object.assign` call not yet supported by most browsers, so you will likely need to include [a polyfill] if you want to use it.

Thanks for sending in this PR!