Update instructions for contributors

[Ceridan]

I faced a few issues after going through the instructions in CONTRIBUTING.md. The PR addresses the following issues:

1. make run-seed-codesources requires a valid Github Access Token to be able to access GitHub data, so it should be added to .env file. I added this information to the CONTRIBUTING.md file. Also, I converted it to a numbered list to make it clear that obtaining the token is a separate step.
2. By default make run-seed-codesources will fail even with the valid token, because when the postgres is started no speedtyper database was created. Fixed it in docker-compose.yml.