docs: expand Once section in README with GHCR context

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: markhallen

README.md

@@ -222,7 +222,7 @@ kamal deploy
 
 ## Once Platform
 
-This app is compatible with [37signals' Once](https://once.com/). See [docs/ONCE.md](docs/ONCE.md) for setup details.
+This app is compatible with [37signals' Once](https://once.com/) for self-hosted, single-tenant deployment. Docker images are published to GHCR on each release and can be installed directly from the Once installer. See [docs/ONCE.md](docs/ONCE.md) for setup details.
 
 ## Docker