Add has child query doc.

docs/Query/HasChild.md

+# Has child Query
+
+> More info about has child query is in the [official elasticsearch docs][1]
+
+The has child query accepts a query and the child type to run against, and results in parent documents that have child
+docs matching the query.
+
+## Simple example
+
+```JSON
+{
+    "has_child" : {
+        "type" : "blog_tag",
+        "query" : {
+            "term" : {
+                "tag" : "something"
+            }
+        }
+    }
+}
+```
+
+In DSL:
+
+```php
+$termQuery = new TermQuery('tag', 'something');
+
+$hasChildQuery = new HasChildQuery('blog_tag', $termQuery);
+
+$search = new Search();
+$search->addQuery($hasChildQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-has-child-query.html