README needs to document relationships among directory options

[dabrahams]

We have directory, root_dir, and working_dir, but we don't know what they are computed relative to, or what the default values are. That makes it hard to know how to set this up.

[rohan-at-sentry]

@thomasrockhu-codecov - this is referencing docs on the readme, which is specific to the options on the GH Action