add $_index to docs

Author: atanas-dev

documentation/30-complex-field/5-config-methods.md

@@ -68,7 +68,11 @@ Allows for a [Lodash template](https://lodash.com/docs/4.17.4#template) to be us
 
 The passed `$template` can also be a [callback](http://php.net/manual/en/language.types.callable.php).
 
-___Note:___ Dashes in field names must be replaced with underscores. For example `field-name-with-dashes` must be referred to as `<%- field_name_with_dashes %>` in `set_header_template()`.
+___Note:___ Dashes in field names must be replaced with underscores.
+Example: `field-name-with-dashes` must be referred to as `<%- field_name_with_dashes %>` in `set_header_template()`
+
+___Note:___ The group index is available as `$_index` (0-based).
+Example: `->set_header_template('This is group <%- $_index %>')`
 
 Example usage: