Add Colonnade to "Who is using Rig?"

Hi, we're using Rig in production and we're impressed!

Could we be listed?

Our app is live at https://colonnade.ai

Suggested line:
- [Colonnade](https://colonnade.ai) - Simple AI chat for everyday use. `rig-core` and `rig-agent` power agent mode.

Thanks for your work in this project 🫶

Find the 'Who is using Rig?' documentation file, likely README.md or a similar markdown file in the repository root. Add the provided line for Colonnade in the appropriate list format. Verify the change by checking the rendered markdown locally or in a preview.

Vom Indexierungsmodell aus dem Issue-Text verfasst.