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: https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard/issues/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.