Fix README, add Synthea seeds, add mock model

[katy-sadowski]

This PR contains:

• Some fixes to the README setup instructions, for clarity
• Adds seed files & configuration for a small Synthea database we'll use for testing. Technically dbt tells us not to use seeds for raw data, but I think this is the easiest way for us all to stay in sync with the data we're using to develop the ETL 😄
• Adds an initial model just for the purpose of testing a dbt run all the way through, to make sure all developers are able to successfully set up the project