Update the docs to clarify functionality

[willhlaw]

Motivation from https://github.com/willhlaw/node-firestore-backup-restore/issues/3#issuecomment-359745241.

• [ ] Clarify operations to retrieve and backup to disk (backup), retrieve and restore to another firestore (clone), and push from backup to another firestore (restore).
• [ ] Add new commands that better match the intended action
• [ ] Deprecate current commands

[jeremylorino]

@willhlaw please assign. i'll pick this up

[willhlaw]

@jeremylorino, that would be great. You can go ahead and submit a PR if you'd like. If you're interested in helping out, then I'll add you as a collaborator to the project and then you'll be able to get assigned.