update persistedStateReducer API

Summary:
upadting to the new API:
```
static persistedStateReducer: ?(
  persistedState: P,
  method: string,
  data: Object,
) => $Shape<P>;
```

Reviewed By: jknoxville

Differential Revision: D10466918

fbshipit-source-id: 08ba963c6ae744d8d445e375c41f601e9bafd4da

docs/writing-a-plugin.md

@@ -39,6 +39,7 @@ static defaultPersistedState = {
 
 static persistedStateReducer = (
   persistedState: PersistedState,
+  method: string,
   newData: Object,
 ): PersistedState => {
   // Logic to merge current state with new data