We need to add a contributing.md file to this repo. This is an extension of this ticket: mesa/mesa#963 (comment), but it can be broken out separately.

AC:

• A contributing.md file that helps people contribute
• Update main Mesa docs as well as contributing.md in the main mesa & mesa-geo repos to remove anything that might be confusing and also add whatever needs to be added for clarity.
• include stuff related to the outcome of discussion on pinning models - Examples: To pin mesa version or not to pin? mesa#1530
• Add some text about writing a readme for an example should be formatted / what it should include