Design philosophy and project goals documentation

- Project goals:
- As close to pathlib as possible; extra properties/methods for critical cloud-specific functions
- Support for different backends is easy to implement
- Only download files locally when necessary
- Local caches can be used across processes (not just temporary dirs with the process)

- Non-goals:
- Interact with non-file management cloud services (databases, etc.)
- Provide permissions management—this is best done by using the cloud service's API directly and managing the differences is more risk that reward
- Syncing local/cloud folders; we only manage our cache, cloud always wins or errors, deleting local files will not remove from cloud