Document how to use the caught exception.

[Wilfred]

It turns out that browsers can help you view a backtrace from a caught exception, but this only seems to be documented in this GitHub thread https://github.com/facebook/react/issues/4216#issuecomment-115053221

Add instructions for the different ways users can take advantage of us throwing an error.

[ghost]

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[zpao]

This seems fine to me. Once you sign the CLA we can get this merged.

[Wilfred]

@zpao I've signed the CLA! :)

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