Clarify that an existing reducer is needed

Fixes wildlifela#9

README.md

@@ -4,26 +4,38 @@ Migrate redux state between versions with redux-persist.
 
 #### Usage
 ```js
-import { compose, createStore } from 'redux'
+import { compose, createStore, combineReducers } from 'redux'
 import { persistStore, autoRehydrate } from 'redux-persist'
 import createMigration from 'redux-persist-migrate'
 
+// VERSION_REDUCER_KEY is the key of the reducer you want to store the state version in.
+// You _must_ create this reducer, redux-persist-migrate will not create it for you.
+// In this example after migrations run, `state.app.version` will equal `2`
+const VERSION_REDUCER_KEY = 'app'
+
+// This is a list of changes to make to the state being rehydrated.
+// The keys must be integers, and migrations will be performed in ascending key order.
+// Note: blacklisted reducers will not be present in this state.
 const manifest = {
- 1: (state) => ({...state, staleReducer: undefined})
- 2: (state) => ({...state, app: {...state.app, staleKey: undefined}})
+  1: (state) => ({...state, staleReducer: undefined})
+  2: (state) => ({...state, app: {...state.app, staleKey: undefined}})
 }
 
-// reducerKey is the key of the reducer you want to store the state version in
-// in this example after migrations run `state.app.version` will equal `2`
-let reducerKey = 'app'
-const migration = createMigration(manifest, reducerKey)
+const migration = createMigration(manifest, VERSION_REDUCER_KEY)
 const enhancer =  compose(migration, autoRehydrate())
 
+const reducer = combineReducers({
+    [VERSION_REDUCER_KEY]: (state = {}) => state, // This reducer will be used to store the version
+    otherReducer1,
+    otherReducer2,
+    // ...
+})
+
 const store = createStore(reducer, null, enhancer)
 persistStore(store)
 ```
 
-In the above example `migration = createMigration(manifest, 'app')` is equivalent to the more generalized syntax:
+In the above example `migration = createMigration(manifest, VERSION_REDUCER_KEY)` is equivalent to the more generalized syntax:
 ```js
 // alternatively with version selector & setter
 const migration = createMigration(