[Good first issue]: Document member RSVP flow in README

[FrankezeCode]

Summary

README mentions Builder Circles on the roadmap but not the live RSVP → matching → circles flow. Add a short member-facing section for contributors and new users.

What should change?

• Add “Member RSVP & Builder Circles” subsection to README.md
• Steps: RSVP at /meetup/[slug] → organizer runs matching → view group at /meetup/[slug]/circles
• Link to docs/BUILDER-CIRCLES.md

Acceptance criteria

README includes clear 3-step flow
Link to detailed docs
No inaccurate claims (check-in = attendance only)

Checklist of done conditions:

• README section added
• Links valid
• Copy reviewed for accuracy vs current product
• Markdown renders correctly on GitHub

How to test

1. Preview README on GitHub or locally
2. Click doc links
3. Compare steps against docs/BUILDER-CIRCLES.md