docs(readme): add aggregate signature verification example

[0xsecaas]

Adds a usage example to the README demonstrating aggregation and verification of multiple signatures over the same message using fast_aggregate_verify from min_sig. This complements the existing single-signer example and reflects actual usage seen in benchmarks.

[dot-asm]

I'm weary to include this example, because it fails to illuminate the key prerequisite for fast_aggregate_verify, having proofs of possession for the participating public keys.