Update README

It would probably be nicer to give instructions to create a symlink to the LaunchAgent, and the latest version, rather than copying the current one. So
`cp /usr/local/Cellar/d-bus/<version>/org.freedesktop.dbus-session.plist ~/Library/LaunchAgents/`
becomes
`ln -s /usr/local/opt/d-bus/org.freedesktop.dbus-session.plist ~/Library/LaunchAgents/`

(because `/usr/local/opt/d-bus` is a symlink to `/usr/local/Cellar/d-bus/<version>`).

That way there will be no need to update should it ever get changed in a D-Bus update.