write best-practices guide for safe, efficient, and effective usage

To Do: write best-practices guide for safe, efficient, and effective usage

Why:
- so that it's clear how to make safe, efficient, and effective use of device storage
- as that usage results in the most performant and effective outcomes
- because this this library is flexible and convenient enough that it can be accidentally misused

Acceptance Criteria:
- adds README documentation with best practices and scenarios
- includes guidance on how to segment cache keys so that automatic pruning is effective on a per-app-page basis
- includes guidance on _when not_ to use this library
- includes guidance on safe or effective alternatives for cases when this library is not a good fit
- includes clear guidance on when to use this in lieu of service workers or a PWA
- includes clear guidance on the vulnerabilities of IndexedDB caching compared to browser cache API

Start with the repository README and the library's existing IndexedDB caching usage. Expand the README with the requested cache-key segmentation, misuse warnings, alternatives, service-worker/PWA guidance, and IndexedDB-versus-Cache-API vulnerabilities; the acceptance criteria define when the documentation is complete.

Written by the indexing model from the issue text.