Add documentation for instantiating P2SH multisig

[trevinhofmann]

There is documentation available at
http://bitcore.io/api/#Create_a_Multisig_Address but I think it would
also be useful in the address.md document.

[coveralls]

Coverage remained the same at 96.91% when pulling 7fd9e8e on thofmann:master into 970bf12 on bitpay:master.

[braydonf]

LGTM.

One small detail: instances should be lowerCamelCase, and only constructors with UpperCamel.

[trevinhofmann]

Thanks. I agree with lowerCamelCase, but was mostly following the example from here (also found here).

Should this example be changed as well?

Edit: I have added a commit to apply lowerCamelCase in both locations.

[coveralls]

Coverage increased (+0.0%) to 96.91% when pulling 99c88c4 on thofmann:master into 970bf12 on bitpay:master.