Documentation: Document missing methods for data module

[gziolo]

When working on publishing data module to npm, we noticed that some methods aren't documented in the README file:

• setPersistenceStorage
• withRehydration
• loadAndPersist

We should fix it. In addition, we should add a note how to use them in node environment because by default they depend on window.localStorage. Example of how to setup it with store npm module:

import store from 'store';

setPersistenceStorage( store );

I haven't tested it, but in general, it should work as explained by @aduth:

So we default to a broken (error-throwing) state for Node environments where this package is used?

It's probably a larger dependency than we'd want, but something like Store.js is a good abstraction for this sort of thing.

[aduth]

Closing since these functions have since been deprecated and removed.

Alternative / equivalent persistence module is documented here:

https://github.com/WordPress/gutenberg/tree/master/packages/data/src/plugins/persistence