Create an api.php file

Thanks for the patch!

Could you change the following:

+ * @param string $message
+ * The message itself.
+ */

This param is an array. The description is also misleading. You can change it to say it is a $message array, as used in drupal_mail.

Also, please change

function hook_welcome_mail_mail_alter($message) {

to

function hook_welcome_mail_mail_alter(&$message) {

...since you are required to use it like that in a custom module.

Your contrib workflow is very good!

Thanks alot for your patch. Commited now!