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
The second "experimental" felt, to me, a bit repetitive, so I left it out, but i can add it back if it feels unclear still. On a whole that wording is much better than my stab at it.