Add Firebase Documentation

[ascibisz]

What needs to happen?

The cellpack client is heavily dependent on firebase, and yet we don't have great documentation on it. We should add firebase documentation to this github repo, probably as its own markdown file, to alleviate this.

The documentation should include

• How to access the firestore database
• Brief overview of why we're using firebase instead of a different database
• How to upload data to firebase
• Schema descriptions of our different collections
• Retention policy for the different collections (and links to the jobs that clean up the different collections)
• How each of the collections are used (either by the client or other places)

[meganrm]

put in CONTRIBUTING