In order to add a review link to an email you must navigate to the email template or create a new email template.
Next, search for the logo of the review site you would like a review for (i.e Google Reviews, Yelp!, Angie's List). Once you find the best logo you can copy and paste the image into the email template.
You can adjust the size of the image by clicking and dragging one of the white boxes in all four corners.
Keeping the image highlighted, the next step is to click on the chain link in the grey toolbar.
It will ask for the URL, make sure you put the link to the exact page you want to be viewed. Leave the title area blank because the logo is doing that job for us and then click on the blue "Ok" button.
Once you are finished editing the email template click on the blue "Save Template".
Check out how to Create or Edit an Email Template!