Add section about Third-party-Skills

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• https://github.com/decebals/claude-code-java

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• https://skills.sh/teachingai/agent-skills/maven-search

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• https://github.com/making/claude-skills/blob/main/spring-code-standards/skills/spring-code-standards/SKILL.md

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https://github.com/jehna/readme-best-practices