Narrative documentation improvements

[rabernat]

Rechunker has gained a lot of new features fast! 🚀

• New executors. We now support
  • Python
  • Dask
  • Prefect
  • Beam
  • PyWren
• Ability to accept zarr storage options (see #48)
• Soon: Xarray support (see #52)

These options are enumerated in the API Docs. But eventually it would be nice to have some narrative documentation describing a full workflow with different executors.

I'm opening this issue for us to track this non-urgent task.