Add README curl example for webhook events

[spaicycookies]

What does this PR do?

Adds a README curl example that posts a webhook event to POST /webhooks/events, including the x-webhook-provider and x-anchor-signature (HMAC-SHA256 over the raw body) headers used by the current SDK.

How to test?

bun run test — new README webhook curl example suite asserts the example and header names are present.

Checklist

• My code follows the code style of this project.
• I have added tests for my changes.
• I have updated the documentation accordingly.
• I have run bun run test and bun run lint locally.

Issue Reference

Closes #269

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[gemini-code-assist]

Code Review

This pull request adds a new 'Webhook events' section to the README.md file, detailing how to post webhook events with signature verification using curl. Additionally, a new test suite is introduced in tests/readme-webhook-curl.test.ts to ensure the README contains the correct curl command, headers, and signature generation steps. There are no review comments, and we have no feedback to provide.

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