Add palantir-java-format-action into the README as a third-party integration

[abashev]

Before this PR

The README lists several ways to use palantir-java-format, including the Eclipse and IntelliJ plugins, the Gradle plugin, and Spotless integration.

However, it does not mention third-party GitHub Actions that wrap palantir-java-format for CI formatting checks.

After this PR

Document palantir-java-format-action as a third-party integration.

palantir-java-format-action is an open-source GitHub Action that checks Java code formatting in CI using the palantir-java-format native binary, without requiring Java, Maven, or Gradle on the runner. By default it only checks files changed in a pull request or push, and caches the native binary between runs.

This makes it easier for users who want a lightweight formatting check in GitHub Actions CI to discover an existing integration path.

Possible downsides?

This adds a link to a third-party project that is not maintained by Palantir.

[changelog-app]

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Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

Add palantir-java-format-action into the README as a third-party integration

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[abashev]

Hi @CRogers @crogoz 👋 Gentle nudge on this one — it's been open for a couple of weeks.

All CI builds are green and the CLA is signed. A changelog entry is already included (changelog/@unreleased/pr-1706.v2.yml, type: improvement). The only things still pending are docs/changelog and policy-bot: develop, which as far as I can tell are waiting on a maintainer review.

The change is small and docs-only: it adds palantir-java-format-action to the README under a new "GitHub Action" section, clearly marked as a community-maintained, third-party integration not officially supported by Palantir.

Would either of you be able to take a look when you get a chance? Happy to adjust the wording or placement if anything needs changing. Thanks!