Add a section about multi-sig

[GBKS]

This has been requested by the community and would make for a great extension to a new security section that's referenced in one of the other issues.

Multi-sig means that the signing of a message or transaction is performed by more than one piece of software, for enhanced security. This could mean the the same user needs to sign it via multiple pieces of software or multiple devices, or even that multiple people are required to sign it.

Since this is more of an advanced topic, it would be helpful to explain the use cases well.