docs: clarify local image support in conditions.uri

[Umanginigam]

Summary

Document support for local images in conditions[].uri for FL2VA examples.

Motivation

Issue #40 asks whether the image URL can be replaced with a local image for testing. The current README only demonstrates remote CDN URLs, while the SGLang MiniMax-H3 deployment also supports file:// URIs.

Changes

• Added a note explaining that file:// URIs are supported.
• Included an example using a local keyframe image.
• Clarified that the path is resolved by the SGLang server process.
• Linked to the SGLang cookbook for more details.

Fixes #40

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