add documentation

`README.md` file describes
- aim and background of the project (including the workflow DAG representation)
- how to install requirements (including setting up a `conda` environment for the project)
- how to execute the workflow run integration test
- how to run the workflow on your own samples (including how to auto-generate required params from LabKey metadata)

Additional minor changes:
- minor changes in various test and related files, including updates of paths
- root directory now includes subdirectory `runs/` for a user's workflow runs (contents not version-controlled)