You can do this by using Captive Portal.
1. Set captive portal to no authentication.
2. You need to allow your website through captive portal with "Allowed IP Addresses"
3. Upload a custome captive portal page with one of the following:
Custom page that looks like your website
A PHP or meta tag redirect
A frame or iframe to you website
Simple page that says what you want to say with a link to your website.
If you will have many users you might want to do the following to help Captive Portal scale better.
These performance improvements I have identified by running 100+ customers in Captive Portal with pfSense.
This prepares the pfSense built in web server for more concurrent traffic.
http://forum.pfsense.org/index.php/topic,8861.msg50280.html#msg50280
This helps optimize PHP so it doesn't hold web server resources for a long period of time.
http://forum.pfsense.org/index.php/topic,8878.0.html
The above performance enhancements will be included in pfSense 1.3.