Explicitly state in the README that storing data in session is unsafe
``` javascript
    try {
      yield* next;
    } catch (err) {
      throw err;
    } finally {
      commit(this, this._prevjson, this._sess, opts);
    }
```

This code will break under race conditions.
You should state it explicitly in the documentation so that users don't rely on the session store to be consistent.
