generating P2SH (2-of-3) multisig address

[watchmyshow]

Summary

There's currently no short example that demonstrates how to generate
a simple P2SH 2-of-3 multisig address on testnet.

Proposal

Add example-generate-multisig-address.js under /src/ showing:

• creation of 3 public keys
• use of payments.p2ms and payments.p2sh
• output of resulting testnet P2SH address

Benefit

Helps developers understand multisig address creation quickly
without reading through complex transaction examples.