Go to http://towelroot.com/ on your Relay and click the picture in the middle of the page, it will download an apk to your device.
 Install the apk you just download (you may have to allow unknown sources in settings)
 Click the make it rain button and wait 15 seconds. Your device will now reboot
 You are now rooted!
 Optional: Go to the Play Store or this thread and download the latest version of SuperSU.
 Optional: Launch the SuperSU app and follow the instructions there. If you do not have a custom recovery installed, do not choose that option for installation.
If you are still having trouble, follow this video. It is for the S5, but it is done exactly the same way!