Add Helm 4 installation instructions with --verify=false flag

[Copilot]

Helm 4 requires plugin verification by default. Installation fails without --verify=false since this plugin doesn't provide provenance artifacts yet.

Changes

• Added Helm 4 installation section with --verify=false flag requirement
• Included links to Helm 4 verification and provenance documentation

# Helm 4 users must use:
helm plugin install https://github.com/databus23/helm-diff --verify=false

Original prompt

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This section details on the original issue you should resolve

<issue_title>Helm 4 Plugin Install requires provenance or --verify=false</issue_title>
<issue_description>Using Helm 4, helm plugin install https://github.com/databus23/helm-diff fails:

$ helm plugin install https://github.com/databus23/helm-diff
Error: plugin source does not support verification. Use --verify=false to skip verification

The workaround is obvious, but perhaps either the README should be updated with --verify=false or the releases should be update to create appropriate provenance artifacts.

See Also

• https://helm.sh/docs/overview
• https://github.com/helm/community/blob/main/hips/hip-0026.md
• https://helm.sh/docs/topics/provenance/</issue_description>

<agent_instructions>add tips in readme.md</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@yxxhero @rrrix got it.

• Fixes Helm 4 Plugin Install requires provenance or --verify=false #885

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