Add video format guidance to README

[bseverns]

Motivation

• Provide clear, practical guidance for the video file used by the sketches because Processing’s Movie can choke on some codecs and cause confusing behavior.
• Reduce runtime stutter and dropped frames by recommending sensible defaults for codec, resolution, and file size.
• Make it easier for users to get the example sketches running smoothly without deep troubleshooting.

Description

• Updated README.md in the "Put a video in the data folder" section to add a short, punky quick-spec list.
• Added recommendations for preferred codec (H.264), default resolution (720p with 1080p as optional if your machine is beefy), and to keep file sizes reasonable.
• Called out that Processing’s Movie can choke on some codecs and that dropping resolution is the easiest win when the effect-heavy draw() loop starts to sweat.

Testing

• No automated tests were run because this is a documentation-only change.

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Codex Task