Either you can write the logic in your controller to send the email after merging is completed OR you can send notification from a trigger on delete and update. Because merge is nothing but delete of one record and update of another. https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers_merge_statements.htm
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_triggers_merge_statements.htm