[HDEE]: Contribution Guidelines

[asreimer]

Contribution guidelines needed for:

• PRs (add .github directory with template files)
• Issues (add .github directory with template files)
• Contributions (“CONTRIBUTING.txt” file in the repo)
• Process for accepting contributions

Some helpful information:

• PyHC guidelines (and guidelines in PyHC packages)
• Mozilla Science Lab guide
• “CONTRIBUTING.txt” file to the repo