Add simple query string query doc.

docs/Query/SimpleQueryString.md

+# Simple Query String Query
+
+> More info about simple query string query is in the [official elasticsearch docs][1]
+
+A query that uses the SimpleQueryParser to parse its context
+
+## Simple example
+
+```JSON
+{
+    "simple_query_string" : {
+        "query": "\"fried eggs\" +(eggplant | potato) -frittata",
+        "analyzer": "snowball",
+        "fields": ["body^5","_all"],
+        "default_operator": "and"
+    }
+}
+```
+
+In DSL:
+
+```php
+$simpleQueryStringQuery = new SimpleQueryStringQuery(
+    '"fried eggs" +(eggplant | potato) -frittata',
+    [
+        'analyzer' => 'snowball',
+        'fields' => ['body^5', '_all'],
+        'default_operator' => 'and',
+    ]
+);
+
+$search = new Search();
+$search->addQuery($simpleQueryStringQuery);
+
+$queryArray = $search->toArray();
+```
+
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-simple-query-string-query.html