Document the wild hostname(1) issue on OS X

As described in #82, the hostname on OS X can vary:

- The result of hostname(1) can change depending on DHCP settings; by
  default, as you move between networks, your hostname will change.
- The value for `ComputerName` as set in scutil(8) contains values that
  are inappropriate for a directory name (spaces, quotes, and so on),
  and is blank by default.
- The value for `LocalHostName` changes based on DNS and DHCP settings.

Therefore, OS X users are highly encouraged to set a hostname using the
`HOSTNAME` variable in rcrc(5).