Add query string query doc.

docs/Query/QueryString.md

+# Query String Query
+
+> More info about query string query is in the [official elasticsearch docs][1]
+
+A query that uses a query parser in order to parse its content.
+
+## Simple example
+
+```JSON
+{
+    "query_string" : {
+        "default_field" : "content",
+        "query" : "this AND that OR thus"
+    }
+}
+```
+
+In DSL:
+
+```php
+$queryStringQuery = new QueryStringQuery('this AND that OR thus');
+$queryStringQuery->addParameter('default_field', 'content');
+
+$search = new Search();
+$search->addQuery($queryStringQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-query-string-query.html