Add CONTRIBUTING.md with GitHub contribution graph troubleshooting

[Copilot]

User reported contribution graph not updating after rewriting commit history. This is a GitHub configuration issue, not a code problem—commits require verified emails, must be on the default branch, and history rewrites need force-pushing.

Changes

• Added CONTRIBUTING.md with:
  • Contribution graph requirements: verified email, default branch commits, force-push after rebase
  • Common issues: past-year limitation, indexing delays, private repo settings
  • Git configuration setup for proper commit attribution

Key points from the guide:

# Verify email matches GitHub account
git config user.email

# After rewriting history, force-push is required
git push --force origin <branch-name>

Note: Old commits (2020) won't appear on current contribution graph—GitHub only displays the past year.

Original prompt

I have rewritten the commit history but my contribution graph is still not updated

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