Describe differences to directly working with the File System Access API in README

Hi :D If you're ok, I'd like to have a small PR to add a small section to the README regarding the migration from the native file system.

Thing is, I used the native file system first, then as my app needs to go live, I adopt this library, and there are several things I learned that I think would be useful for others in similar situation.

For example:
- In the native API, the main model is "handle", and you get the file content from there, async
- In this API, the main model is the file itself, and the "handle" is an optional object
- Stuff like persisting to local storage (indexed DB actually)

Start with the README and review the issue's listed differences between the native File System Access API and this library. Add a migration section covering handles versus files, asynchronous content access, and IndexedDB persistence; done means the section clearly helps native-API users understand the transition.

Written by the indexing model from the issue text.