README.md is missing necessary guidance / updates

[JakeUrban]

There were a couple issues I ran into trying to follow the README:

• I got errors about using an incompatible stellar-core version. I resolved this by updating the image specified in docker-compose.yaml to stellar/stellar-rpc rather than stellar/soroban-rpc.
• Some environment variables are required but probably shouldn't be for local development (TRACKER_DSN), and others probably deserve some guidance on how to get appropriate values (CHANNEL_ACCOUNT_ENCRYPTION_PASSPHRASE, DISTRIBUTION_ACCOUNT_PRIVATE_KEY, etc.)

I think the README is also missing some key info that would help contributors:

• How to run tests on their machine (ideally w/o docker)
• Referencing the API documentation in /openapi
• Documentation for the supported environment variables (or at least the required ones)