There is already a Read More button for the Classik theme portfolio. Check out the demo – http://demo.appfinite.com/classik/portfolio/
To get it to work, you’ll need to go to your Genesis Theme Settings, scroll to the bottom for the Portfolio Options and change “Select one of the following” to “Display Post Content” and then add the content limit….then it should show up.
If you need to change the Read More text to say something different, you’ll need to edit this in your page_portfolio.php file of the theme.