Workflow API usage examples

This project would benefit having some examples in the README showing how this API is expected to be used.

This is also particularly relevant because the workflow execution request object contains many fields with a generic name. It's not easily understandable how to specify the workflow to be executed, the required input/output objects and any other required data.

There's a small example in the [reference implementation](https://github.com/common-workflow-language/workflow-service/tree/ga4gh-wes) project but it doesn't seem to be aligned with the current schema definition.