README: add info about CI / GitHub action

[metachris]

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[claude]

Claude finished @metachris's task in 33s —— View job

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This PR adds a "CI / GitHub Actions" section to the README documenting the flashbots/builder-playground-action. The change is straightforward documentation — 27 lines of markdown with two YAML examples.

No issues found. The section is well-placed (after Installation, before Getting Started), concise, and shows two practical examples covering the common use cases (basic usage and pinning a version with a custom recipe). The formatting is consistent with the rest of the README.

One minor note: I wasn't able to verify that the flashbots/builder-playground-action repo exists and that the documented inputs (recipe, version) match the actual action definition, due to tool permission constraints. Worth double-checking that the examples reflect the current action interface.