Update guidance on accepting extensions to the partner repo

[richard-cox]

• Add to https://github.com/rancher/partner-extensions README
• Similar to https://github.com/rancher/partner-charts/tree/main-source?tab=readme-ov-file#what-are-the-requirements-for-adding-a-project-to-this-repository
• Might include
  • demonstrate to us that the extension works
  • tell us how you've tested it
  • tell us what version of Rancher components you've tested it with
  • make sure those are documented
  • UI/UX brief review (for breaking changes)