Improve binding patterns for arrow functions

README.md

@@ -499,6 +499,9 @@ can pass additional arguments that control the code output:
   - `3` -- always use the original quotes
 - `keep_quoted_props` (default `false`) -- when turned on, prevents stripping
   quotes from property names in object literals.
+- `es` (default `5`) -- set output printing mode.  This will only change the
+  output in direct control of the beautifier.  Non-compatible features in the
+  abstract syntax tree will still be outputted as is.
 
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