docs: update README with Nakamoto, sBTC, and Clarity 3 information

[sinirlibiber]

Description
Applicable issues
fixes #
Additional info (benefits, drawbacks, caveats)
Checklist
Test coverage for new or modified code paths
For new Clarity features or consensus changes, add property tests (see docs/property-testing.md)
Changelog is updated
Required documentation changes (e.g., rpc/openapi.yaml for RPC endpoints, event-dispatcher.md for new events)
New clarity functions have corresponding PR in clarity-benchmarking repo

(#6991)