Update README with note on repo update frequency

[teempai]

Added note about update frequency and repo maintenance.

Summary

Describe the user-visible change and why it is needed.

Security boundary

Describe any effect on authentication, pairing, encryption, relay behavior,
project authority, provider execution, approvals, Keychain access, or stored
context. Write No boundary change only after checking SECURITY.md.

Verification

• npm run typecheck
• npm test
• npm run build
• Relevant Swift tests/builds
• Failure paths and user-visible errors tested
• Documentation updated when behavior or assumptions changed
• No credentials, private data, personal paths, or raw conversations included