Add instructions to README.md on how to run the watch app via Buck CLI

[bachand]

I spent a few minutes on this and I can't figure it out.

I ran make debug, and then tried opening a Watch simulator via the "Hardware" menu of the simulator, but it wasn't paired.

I then tried running xcrun simctl list to see if I could specify a paired iPhone + Watch simulator, but I wasn't sure how to specify one on the command line.

Ideally there would be a command to do this in the Makefile if that's possible.

[dfed]

We should be able to use xcrun to create a device pair. Might be a bit of a pain to do programmatically (since IIRC we’ll need to grab the output of xcrun, grab device UUIDs, then run a separate command to create a pair), but it should be possible.

This post should prove helpful as we try to get this running. As a note to myself (and nerd-snipe for the interested) I’m also guessing if we can solve this here, we can probably use the fix to resolve square/Valet#128.