Document ponyup macOS/brew libressl connection

[SeanTAllen]

#117 was caused by this.

macOS, it's all dynamic linking.

brew install libressl will periodically change the version it installs as they switch to a newer version of libressl. This is rare, but does happen. When that happens, older versions of ponyup will stop working once libressl is updated.

We need to document the expected error that users would see and tell them to reinstall ponyup via the init script as that will download the most recent nightly version. Within 24 hours of a libressl change, it will work again.

This also means, that eventually if they update to a different version of ponyup, that it might be using a different libressl than they have installed and will fail.