Document Stripe webhook setup for membership lapsing

[conatus]

Summary

• Expands the existing "Membership Lapsing" section in readme.md with concrete setup instructions
• Documents the webhook endpoint URL, the five Stripe events to subscribe to, and the Lapsed Tag plugin setting
• Explains the lapsing/unlapsing logic (which statuses trigger each)

Test plan

• Review the rendered Markdown in the PR preview
• Confirm endpoint URL, event names, and setting name match the current codebase