docs(readme): clarify custom agent model syntax

[LaishGlenberg]

Addresses #175

Was not sure where in the readme to put this, eventually settled on adding it to the end before License.

Was not sure if this belonged under resources/walkthrough or readme, but felt it was relevant enough to be added into readme (when I reviewed documentation I did not realize there was a walkthrough section at first). Apologies if this was incorrect.

No hard feelings if the PR is denied, always open to feedback so I can improve future contributions. Tried my best to match your internal commit standards and markdown/writing style.

Cheers!

[Vizards]

Thanks for putting this together — this is a useful usage note.

Would you be willing to move this into a new docs/FAQ.md and organize it in a Q&A format? I think the custom-agent model syntax would fit well as the first entry, and the file could become the home for similar usage questions going forward.

For example:

## How do I select a DeepSeek model in a custom agent?

...

Please also add docs/FAQ.md to .vscodeignore so the standalone FAQ is not bundled into the VSIX.

If you would rather not make another round of changes, I am happy to adjust the PR myself and add you as a co-author on the resulting commit. Let me know which you prefer.