Add query filter doc.

docs/Filter/Query.md

+# Query Filter
+
+> More info about query filter is in the [official elasticsearch docs][1]
+
+Wraps any query to be used as a filter.
+
+## Simple example
+
+```JSON
+{
+    "filter" : {
+        "query" : {
+            "query_string" : {
+                "query" : "this AND that OR thus"
+            }
+        }
+    }
+}
+```
+
+And now the query via DSL:
+
+```php
+$queryStringQuery = new QueryStringQuery('this AND that OR thus');
+$queryFilter = new QueryFilter($queryStringQuery);
+
+$search = new Search();
+$search->addFilter($queryFilter);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-filter.html