Inside your main app, all you have to do is to add a new file at this location: /app/views/locomotive/devise_mailer/reset_password_instructions.html.haml.
After a restart of your server, Devise should use this file as the new template for the reset password instructions.
We did that for LocomotiveHosting.com (which is built on top on the LocomotiveCMS engine) and it works well.
Thanks!
Did