Create CONTRIBUTING.md file

[joshmedeski]

As a maintainer of an open-source project, I want a clear and helpful CONTRIBUTING.md document that explains how to contribute to the project, so that I can set expectations and explain how I'd like new code the be added to the project.

• Explain the values of the project
• Explain how to add a new icon (alphabetically)
• Explain the single responsibility principle (one change at a time, please)
• Explain that git commits don't really matter (we squash merge)
• Explain that new features should be discussed and agreed upon before a pull request will be accepted