Allow multiple register => strings in the sip.conf

As it was thought of in the initial request to allow setting register => line in sip.conf, 
this patch allows to set multiple register => strings in sip.conf ;)

It's backward compatible with what is there right now. 
The user either specifies the registration string, or an array of registration
strings.
Depending on either Array or String, the template takes care.

While there, added some documentation to the README.md about that
parameter.

cheers,
Sebastian