Add missing documentation for isSaved property

Author: kiaking

docs/api/response.md

@@ -20,6 +20,12 @@ The Response object is what gets returned when you make API call via Request obj
 
   The result of Vuex ORM persistent method.
 
+### isSaved
+
+- **`isSaved: boolean`**
+
+  Whether the response data is persisted to the store or not.
+
 ### model
 
 - **`model: typeof Model`**

docs/guide/usage.md

@@ -117,3 +117,17 @@ if (result.response.data.error) {
 // Persist in the store otherwise.
 result.save()
 ````
+
+You can check to see if the response data is already stored in the store or not with `isSaved` property.
+
+```js
+const result = await User.api().get('/api/users', {
+  save: false
+})
+
+result.isSaved // <- false
+
+result.save()
+
+result.isSaved // <- true
+```