Added missing modernize to README

Author: DreierF

README.md

@@ -42,6 +42,7 @@ plugins: [
 | `sourceMap` | `boolean\|false` | Whether or not to produce source maps for the given input. |
 | `minify` | `boolean\|false` | Minify using Terser, if turned off only comments will be stripped. |
 | `downlevel` | `boolean\|true` | Produces a bundle for `nomodule` browsers. (IE11, ...) |
+| `modernize` | `boolean\|true` | Attempt to upgrade ES5 syntax to equivalent modern syntax. |
 | `verbose` | `boolean\|false` | Will log performance information and information about polyfills. |
 | `polyfillsFilename` | `string\|polyfills.legacy.js` | The name for the chunk containing polyfills for the legacy bundle. |