Document kext fix.

ping @SvenDowideit @steeve  PTAL

I'm not sure where you want this, so I took a swing in the dark.  At any rate, after running into the kext / OSX update issue for the billionth time and finally remembering the esoteric command to fix this, I decided to document it.

I'm also happy to implement this as a boot2docker cli command if you want (`kext-reload`?).