Improve documentation on Storage Engine

* Improve documentation about RemoteStorage implementation.

Author: yookoala

configuration-options/game-configuration/storage-engine.md

@@ -36,3 +36,128 @@ The session storage works just like the local storage engine with the important
 
 ## Remote Storage
 
+For remote storage, your configuration in `options.js` can be set as follow.
+
+```javascript
+    'Storage': {
+        'Adapter': 'RemoteStorage',
+        'Store': '',
+        'Endpoint': 'https://foobar.com/gameSave'
+    }
+```
+
+Or you may add a store name:
+
+```javascript
+    'Storage': {
+        'Adapter': 'RemoteStorage',
+        'Store': 'MyGame',
+        'Endpoint': 'https://foobar.com/gameSave/'
+    }
+```
+
+The storage engine concats the `Store` and `Endpoint` values as the actual RESTful API **endpoint url** to use:
+
+```
+https://foobar.com/gameSave/MyGame
+```
+
+### Implementing the Remote Storage
+
+The storage mimic that of a localStorage, which is simple JSON object storage.
+
+The **endpoint url** should implement the below API interface:
+
+---
+
+```http
+GET [endpoint url]/
+```
+
+{% hint style="info" %}
+Normally, retrieves all the values in the space in a key-value JSON object. If previous storage does not exist, return an empty JSON object: `{}`.
+
+In keys-listing mode, return all keys stored in the space as a JSON array.
+{% endhint %}
+
+**Parameters**
+
+| Name | Description |
+| ---- | ----------- |
+| keys | Boolean. If set to `true`, will switch to keys-listing mode. |
+
+---
+
+```http
+DELETE [endpoint url]/
+```
+
+{% hint style="info" %}
+Clear the entire storage object.
+
+Equivlant to
+```javascript
+localStorage.clear();
+```
+{% endhint %}
+
+---
+
+```http
+GET [endpoint url]/{key}
+```
+
+{% hint style="info" %}
+Return the value of the key in the storage object.
+
+Equivlant to
+```javascript
+localStorage.getItem(key);
+```
+{% endhint %}
+
+---
+
+```http
+POST [endpoint url]/{key}
+```
+
+{% hint style="info" %}
+Store the value in the storage object under the key.
+
+The request body will be the JSON value to store.
+
+Equivlant to
+```javascript
+localStorage.setItem(key, value);
+```
+{% endhint %}
+
+---
+
+```http
+PUT [endpoint url]/{key}
+```
+
+{% hint style="info" %}
+Update a key-value pair. In difference with the set () method, the update
+method will use an Object.assign () in the case of objects so no value is
+lost.
+
+The request body will be the JSON value to update the value with.
+{% endhint %}
+
+---
+
+```http
+DELETE [endpoint url]/{key}
+```
+
+{% hint style="info" %}
+Return the value of the key in the storage object.
+
+Equivlant to
+```javascript
+localStorage.removeItem(key);
+````
+{% endhint %}