Add helm documentation about GitHub App secrets

[eleftherias]

Summary

Update helm README with instructions on github app secrets.

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 47.728% (+0.01%) from 47.716%
when pulling c01f854 on eleftherias:helm-github-app-docs
into 809687a on stacklok:main.

[jhrozek]

I was wondering why this disapperared from one of the renders