feat: show how to use mnemonic and storage

[coolaj86]

A better example of how to use storage and mnemonic that puts storage and mnemonic code in ./lib/ as to make the example completely useful, but still simple at-a-glance.

Would replace #4

[coolaj86]

Also, package.json.browser can be used to provide code samples for both Node.js and Browsers. For example:

{
  // ...
  "browser": {
    "./lib/storage.js": "./lib/browser-storage.js"
  },
  // ...
}

This would use ./lib/storage.js for Node.js, but ./lib/browser-storage.js in a browser (via bundlers). Then you can provide complete, usable examples that Just Work™ no matter the environment.

[markin-io]

The storage is currently under refactoring and the way we do it might become obsolete.
We'll revisit this piece of the documentation later once the change is done.

[coolaj86]

Better maybe obsolete and working now than not working now and possibly just fine. In the browser you're stuck with a key/value store (localForage to get beyond the 10mb localStorage limit)... unless you get really crazy.

Might as well make the API something that either works relatively well with a key/value store, or that easily wraps a key/value store.

If the former, nothing breaks.

If the later, things are fixed with a one line change: newStorage = dapi.wrapKeyValue(oldStorage).