chore(root): add MailerSend to integrations list in readme

[ibrahim123-sia]

The mailersend example exists under examples/mailersend but was missing from the Integrations list in the root README.

---

Summary by cubic

Add MailerSend to the Integrations list in the root README to match the existing examples/mailersend. Ensures the docs are consistent and helps users find the MailerSend example.

^{Written for commit 3c35b01. Summary will update on new commits.}

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3c35b01

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[vercel]

@ibrahim123-sia is attempting to deploy a commit to the resend Team on Vercel.

A member of the Team first needs to authorize it.

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Documentation-only change: adds missing MailerSend entry to integrations list in README. No logic, code, or infrastructure impact.}