Add requirements for self-hosted GitLab instances

[kjenney]

🚨 Proposed changes

A couple of updates to the documentation after setting up the plugin with my own self-hosted GitLab instance.

⚙️ Types of changes

What types of changes does your code introduce? Put an x in the boxes that apply

• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x ] Documentation Update (if none of the other choices apply)
• Refactor

[antoniomuso]

This is an OR not a if