docs(readme): add 'Fetch missing' section

[oscard0m]

Resolves #2500

---

Behavior

Before the change?

Missing information about 'Fetch Missing' in README.md

After the change?

Added information about 'Fetch Missing' in README.md

Other information

---

Additional info

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [ ] Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes (Please add the Type: Breaking change label)
• [x] No

If Yes, what's the impact:

• N/A

Pull request type

Type: Documentation

---

[wolfy1339]

Docs look good, the error message should be changed in @octokit/request to match here

EDIT: https://github.com/octokit/request.js/pull/617

[github-actions[bot]]

:tada: This PR is included in version 3.1.0 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: