Explain accessing local/sync storage directly

Add description from #2 about so future readers can find this info quickly.

README.md

@@ -47,8 +47,36 @@ export default () => {
 }
 ```
 
+### Accessing local/sync storage directly
+
+react-redux prefixes storage keys with `"persist:" + config.key`. If code needs to access storage
+directly instead of through Redux, it can construct the key and access the APIs like the following:
+
+```js
+// Example config passed to `persistReducer`
+const key = 'localStorage';
+const localStorageConfig = {
+  key,
+  storage: localStorage,
+}
+
+// …
+
+chrome.storage.local.get([`persist:${key}`], (items) => {
+  const rootParsed = JSON.parse(items['persist:localStorage']);
+
+  // Keep in mind that each reducer must be parsed separately
+  const someReducer = JSON.parse(parsed.someReducer);
+
+  // `someReducer` will be the contents of the reducer of that name when last persisted
+});
+```
+
+Further discussion: [Accessing storage from content_scripts directly (plain JS) #2][5]
+
 [0]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/storage
 [1]: https://github.com/rt2zz/redux-persist
 [2]: https://www.npmjs.com/package/redux-persist-webextension-storage
 [3]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/storage/local
 [4]: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/storage/sync
+[5]: https://github.com/ssorallen/redux-persist-webextension-storage/issues/2