Added documentation for the simple_query_string

For PR on elasticsearch adapter - feathersjs-ecosystem/feathers-elasticsearch#24

Signed-off-by: Matt <matt@ideajunction.uk>

api/databases/elasticsearch.md

@@ -206,6 +206,26 @@ query: {
 }
 ```
 
+### $sqs
+[simple_query_string](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-simple-query-string-query.html). A query that uses the SimpleQueryParser to parse its context. Optional `$default_operator` which is set to `or` by default but can be set to `and` if required.
+
+```js
+query: {
+  $sqs: {
+    $fields: [
+      'title^5',
+      'description'
+    ],
+    $query: '+like +javascript',
+    $default_operator: 'and'
+  }
+}
+```
+This can also be expressed in an URL as the following:
+```http
+http://localhost:3030/users?$sqs[$fields][]=title^5&$sqs[$fields][]=description&$sqs[$query]=+like +javascript&$sqs[$default_operator]=and
+```
+
 ## Parent-child relationship
 Elasticsearch supports [parent-child relationship](https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-parent-field.html), however it is not exactly the same as in relational databases. feathers-elasticsearch supports all CRUD operations for Elasticsearch types with parent mapping, and does that with the Elasticsearch constrains. Therefore: