[docs] MutationJob docs added

[docs] fl-query for flutter blog added

docs/docs/basics/MutationJob.md

@@ -1,4 +1,26 @@
 ---
 title: Mutation Job
 sidebar_position: 5
----
+---
+
+`MutationJob` is just like [`QueryJob`](/docs/basics/QueryJob) but for `Mutations`.
+It is used to define how & where the new data is inserted or existing data is updated or deleted. Basically, it's the kind of Job that you'll use with http `POST/PUT/DELETE` requests but it doesn't have to be just HTTP requests, it can be anything that returns as long as it returns Future
+
+Here's a simple example:
+
+```dart
+final basicMutationJob = MutationJob<Map, Map<String, dynamic>>(
+  mutationKey: "basic-mutation-example",
+  task: (key, data) async {
+    final response = await http.post(
+      Uri.parse("https://jsonplaceholder.typicode.com/posts"),
+      headers: {'Content-type': 'application/json; charset=UTF-8'},
+      body: jsonEncode(data),
+    );
+    return jsonDecode(response.body);
+  },
+);
+```
+
+Here, instead of a `queryKey` there's a `mutationKey` parameter that is used to identify the Job.
+`Mutation` also supports _retries_ but instead of `externalData` `Mutations` has `variables` parameter that used when the `mutate` method of Mutation is called where you can pass outside data to the `Mutation.task`