Add has child doc.

docs/Filter/HasChild.md

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