docs: Add contributing tutorial

docs: Add contributing tutorial

This adds a tutorial for developing/contributing to OSTree. The following information is included:
- Installing build dependencies for OSTree
- Building OSTree, and running in a container and VM
- Adding a basic command to OSTree, and testing the command
- Suggested git workflows for working with the upstream repo

This is helpful to give new contributors a more detailed introduction to developing OSTree, and an idea of what the workflow looks like.

Closes: #1694
Approved by: cgwalters