Add has parent query doc.

docs/Query/HasParent.md

+# Has Parent Query
+
+> More info about has parent query is in the [official elasticsearch docs][1]
+
+The has parent query accepts a query and a parent type. The query is executed in the parent document space, which is
+specified by the parent type. This filter returns child documents which associated parents have matched.
+
+## Simple example
+
+```JSON
+{
+    "has_parent" : {
+        "parent_type" : "blog",
+        "query" : {
+            "term" : {
+                "tag" : "something"
+            }
+        }
+    }
+}
+```
+
+In DSL:
+
+```php
+$termQuery = new TermQuery('tag', 'something');
+
+$hasParentQuery = new HasParentQuery('blog', $termQuery);
+
+$search = new Search();
+$search->addQuery($hasParentQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-has-parent-query.html