Add git-extend link to External Git Utilities

[nickolasburr]

Description

Add git-extend link to External Git Utilities section.

Type of changes

• A helper script
• A link to an external resource like a blog post

Checklist:

• All new and existing tests pass.
• Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an ok exception)\
• Scripts are marked executable
• I have confirmed that the link(s) in my PR are valid.
• I have read the CONTRIBUTING document.

[unixorn]

Thanks for the contribution

[nickolasburr]

Thank you!