Point OSX users to the fix they need to run before joining agents.

This info about the fix exists on the website, but isn't pointed to in the quickstart. I think a lot of users will be using OSX to try serf out, and it's confusing when it doesn't work as explained in the quickstart.
Merge pull request #60 from dustinmm80/patch-2

Point OSX users to the fix they need to run before joining agents.