chore: add community-maintained extension package explanation

[Myxogastria0808]

The author should do the following, if applicable

This PR adds a short section to the @hono/zod-openapi README introducing a community-maintained extension, zod-openapi-share, which helps centralize and reuse common OpenAPI response definitions.

The extension is clearly marked as non-official and is intended to complement @hono/zod-openapi in real-world API development.

• Add tests
• Run tests
• yarn changeset at the top of this repo and push the changeset
• Follow the contribution guide

[changeset-bot]

⚠️ No Changeset found

Latest commit: 6108140

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