Add Contributing Guidelines

[khos2ow]

Prerequisites

Put an x into the box(es) that apply:

• This pull request fixes a bug.
• This pull request adds a feature.
• This pull request enhances existing functionality.
• This pull request introduces breaking change.

For more information, see the Contributing Guide.

Description

This pull request adds Contributing Guidelines document.

Issues Resolved

List any existing issues this pull request resolves.

Checklist

Put an x into all boxes that apply:

• I have read the Contributing Guidelines.

Tests

• I have added tests to cover my changes.
• All tests pass when I run make test.

Documentation

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Code Style

• My code follows the code style of this project.
• All checks pass when I run make checkfmt and make lint.