Add contributing guide

[izellevy]

Solution

Add a guide for contribution.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [x] Non-code change (docs, etc)
• [ ] None of the above: (explain here)