add webhook handler guide

[cb-karthikp]

Add webhook usage guide in readme

Added README documentation for WebhookEventHandler, including typed event callbacks, webhook handling, optional Basic Auth, fallback behavior, and WebhookResult semantics. Added the handler to the SDK features list.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Enterprise

Run ID: 400e8f6b-1d91-460b-95c7-a2b5ba50f210

📥 Commits

Reviewing files that changed from the base of the PR and between 26d271e and 66b475a.

📒 Files selected for processing (1)

• README.md

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Walkthrough

README.md now documents WebhookEventHandler, including typed callbacks, optional Basic Auth, fallback behavior, request outcomes, and WebhookResult methods. The SDK feature list now includes webhook event handling.

Changes

Webhook handler documentation

Layer / File(s)	Summary
Document webhook handling README.md	Documents typed customer and subscription callbacks, webhook request processing, success and failure handling, optional Basic Auth, fallback callbacks, WebhookResult methods, and the SDK feature-list entry.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Mergeability Score: ⚪ Minimal · up to 66b47

This PR adds webhook usage guidance to the README without changing product behavior or runtime code, so no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 1

✅ Passed checks (1 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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