Add a line of explanation on credentials asked for
Add a line of explanation on credentials asked for

Fixes #21

neonichu/xcode-install,jpsim/xcode-install,lacostej/xcode-install,lacostej/xcode-install,jpsim/xcode-install,sirlantis/xcode-install,KrauseFx/xcode-install,apalancat/xcode-install