docs(readme): add link to Catwalk frontend for provider/model IDs

Amolith created 

Add a reference to the unofficial Catwalk frontend at catwalk-fe.secluded.site
where users can browse available provider and model IDs.

Assisted-by: Kimi K2 Thinking via Crush <crush@charm.land>

Change summary

README.md | 2 ++
1 file changed, 2 insertions(+)

Detailed changes

README.md 🔗 

@@ -56,6 +56,8 @@ All flags are optional if configured in `~/.config/garble/config.toml`.
 | `--model`      | Model ID                                    | `gpt-4o`, `claude-3-opus`, `gemini-1.5-pro` |
 | `--directions` | Instructions for how to transform the input | `"fix grammar and spelling"`                |
 
+Valid provider and model IDs are listed at https://catwalk-fe.secluded.site (an unofficial frontend for Charm's [Catwalk](https://github.com/charmbracelet/catwalk) service).
+
 ### Configuration
 
 Create `~/.config/garble/config.toml`: