allow crossmounts and use subnet mask examples for readme

[dev00]

Just ran into two issues and decided to fix them.

First thing was that setting the IPs via * would not work, so I decided to stick to the netmask example. Imho this is also much more readable.

Second I took your advice by assembling my nfs share by mounting different folders. This didn't work at first - turns out when you allow a whole network, remember to set this option, otherwise you see only empty directories.

Hope this is useful for you :)

[colearendt]

FWIW #23 allows similar behavior by giving you the ability (as the user) to specify arbitrary mount arguments