Add Rainbow Gum to logging list

[agentgt]

As additional verification Rainbow Gum is listed on SLF4J site under the hamburger menu.

Suggestion type

• Project
• Resource

Checklist

• I searched the list and existing issues for duplicates.
• I changed README_SOURCE.md, not the generated README.md.
• This pull request contains one suggestion.
• The suggestion is relevant to Java or the JVM and fits its chosen category.
• I used the canonical project or resource link.
• The suggestion is current and maintained.
• The concise, neutral description explains its distinguishing value and ends with punctuation.
• Licensing is clear and any restrictive terms are disclosed where applicable.

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Summary by cubic

Adds Rainbow Gum to the Java logging libraries list in README_SOURCE.md.

^{Written for commit 6863940. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}