Create SpamAssassin plugin

I require a plugin for SpamAssassin to read an email body (plaintext and html) for words (such as "unsubscribe") and if found prepend the email body with a piece of text. For example, all of my emails flow through SpamAssassin to check for spam, If an email contains an Unsubscribe link i would like to change the top of the email to clearly show that the email contains an unsubscribe link.