Add Contributing.md file & edit related files

We need to add a contributing.md file to this repo. This is an extension of this ticket: https://github.com/projectmesa/mesa/issues/963#issuecomment-1316785220, 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 - https://github.com/projectmesa/mesa/discussions/1530
- [ ] Add some text about writing a readme for an example should be formatted / what it should include

Start by reviewing the existing documentation in the mesa-examples repository and the related Mesa and mesa-geo documentation, then read the linked discussion on pinning models. Create CONTRIBUTING.md with contribution guidance, clarify the related documentation, and document the expected format and contents of example READMEs. Done means all acceptance criteria are addressed across the named repositories.

Written by the indexing model from the issue text.