a quick hack plugin to allow people to sign up for Mailtrain newsletters at WooCommerce checkout.
## Install
1. First, you'll need [CheckoutFieldEditor](https://woocommerce.com/products/woocommerce-checkout-field-editor/) installed and add a field with name `newsletter_`
2. Get your API key from Mailtrain
3. Get your ListID from Mailtrain
4. Edit line 28 in wootrain.php replacing your Mailtrain URL, Mailtrain list ID and mailtrain API key