Save the date of subscribing and provide it as a token

Hello salvis,

First of all thanks for this great module.

I am having difficulties in getting custom value in mail template.

Can I have any documentation for email template from where I can add custom values? I am using Taxonomy Subscription module and I want to have a date on which user has subscribed that taxonomy.

It will be great if there any way or any function from where I can customize email before sending.

Let me know your input.

Thanks in advance.
Anks