Clarify Support For Multiple Projects And Datasets

[eli-b]

Currenly, it's easy to reach the incorrect conclusion that only a single project and dataset are supported:

• The --project flag is required, and only lets you define a single project.
• The --dataset flag also only lets you define a single dataset.
• The data.yaml contains a single project and dataset.

This PR clarifies that multiple projects and datasets are supported:

1. Make the --project flag only required when --data-from-yaml isn't specified.
2. Update the description of the --project, --dataset, and --data-from-yaml flags.
3. Update README.md.
4. Update data.yaml to showcase multiple projects and datasets.

I tested the changes locally and both --project initialization and --data-from-yaml initialization seem to work.