Update docs with example for Babel 7 -- JSX allows elements as attribute values?

[noahlemen]

babel/babel#6004 was resolved via babel/babel#6006, so as of Babel 7 <div attr=<div /> /> will work.

Should an example of this be added in the React documentation? If so, where? Would JSX In Depth be appropriate?

[bvaughn]

Personally I don't think this needs to be in the React docs. (And at least some syntax highlighters- like the Babel plugin for Sublime- don't handle nested elements like this without the wrapping {}s. Atom seems okay with it though.)

But I don't have a strong feeling on this so I'll defer to others.

[gaearon]

I also don't feel this is important enough to document. Note that we already document the pattern itself, just not the form that omits curlies.

[noahlemen]

makes sense, thanks!