added docs for parent inner hit

docs/InnerHits/Parent.md

+# Parent Inner Hits
+
+> More info about inner hits is in the [official elasticsearch docs][1]
+
+The `parent/child` inner_hits can be used to include parent or child. 
+The usage of parent inner hits is very similar to that of nested [inner hits](Nested.md), the only
+difference is that in stead of passing the `path` to the nested object, the parent/child `type` 
+needs to be passed to the `$path` variable. 
+
+## Simple example
+
+```JSON
+{
+    "inner_hits" : {
+        "children" : {
+            "type" : {
+                "article" : {
+                    "query" : {
+                        "match" : {"title" : "[actual query]"}
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$matchQuery = new MatchQuery('title', '[actual query]');
+$innerHit = new ParentInnerHit('children', 'article', $matchQuery);
+
+$search = new Search();
+$search->addInnerHit($innerHit);
+$search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-inner-hits.html