Docs: Update ecmaVersion instructions (#7195)

docs/user-guide/configuring.md

@@ -21,7 +21,7 @@ Please note that supporting JSX syntax is not the same as supporting React. Reac
 
 Parser options are set in your `.eslintrc.*` file by using the `parserOptions` property. The available options are:
 
-* `ecmaVersion` - set to 3, 5 (default), 6, or 7 to specify the version of ECMAScript you want to use.
+* `ecmaVersion` - set to 3, 5 (default), 6, 7, or 8 to specify the version of ECMAScript syntax you want to use. You can also set to 2015 (same as 6), 2016 (same as 7), or 2017 (same as 8) to use the year-based naming.
 * `sourceType` - set to `"script"` (default) or `"module"` if your code is in ECMAScript modules.
 * `ecmaFeatures` - an object indicating which additional language features you'd like to use:
     * `globalReturn` - allow `return` statements in the global scope