Update README

Author: donnguyen

README.md

@@ -215,7 +215,7 @@ end
 
 `cond` can be :
 
-* `:like`, `:start_with`, `:end_with`, `:string_eq` for string or full text search
+* `:like`, `:start_with`, `:end_with`, `:string_eq`, `:string_in` for string or full text search
 * `:eq`, `:not_eq`, `:lt`, `:gt`, `:lteq`, `:gteq`, `:in` for numeric
 * `:date_range` for date range (only for Rails > 4.2.x, see [here](#daterange-search))
 * `:null_value` for nil field