Add is multi option to select docs.

Author: Hyperkid123

packages/react-renderer-demo/src/pages/provided-mappers/select.js

@@ -21,6 +21,10 @@ const schema = {
 
 const variants = [
   ...baseFieldProps,
+  {
+    name: 'isMulti',
+    type: 'boolean'
+  },
   {
     name: 'options',
     type: 'array'