Add docs for the multiple proxy object syntax

[jbranchaud]

Related to issue #3348

I'm documenting how I understand this feature is supposed to work. I have not been able to successfully utilize this syntax in my own local CRA app. If there is anything obvious that I'm missing, I'd appreciate a pointer in the right direction.

[gaearon]

Sorry, I messed up git history. Would you mind doing

git fetch origin

Then write down hashes of your commits, do git reset --hard origin/master and cherry-pick them back onto your branch.

[jbranchaud]

@gaearon I cleaned up the commit history

[stale]

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[stale]

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