Add note about reattach-to-user-namespace when trying to use Xrun with tmux

[gfontenot]

if you have tmux misconfigured, xcrun instruments isn't able to communicate with the simulator (?) properly. We should probably add a note in the async build section of the README and in the Xrun section of the documentation about this.

/cc @sharplet

[keith]

Marginally related: ChrisJohnsen/tmux-MacOSX-pasteboard#43

Mainly just showing that even though everything is maybe suppose to work now, it still doesn't.

[keith]

And with Sierra this is all broken again.

[keith]

FWIW the issues I was seeing related to this are fixed with a Sierra update. It might be the case for the original issue here too.