Open Settings -> About Phone -> tap build number about 7 times until you are a developer.
Back in Settings -> Developer Options -> Root Access -> Apps and ADB (recommended)
After that you should be able to install SuperSU from the Play Store and grant it superuser permissions to allow it to install the binary.
Alternatively to all this you can download the SuperSU zip file from here and flash it in recovery. It will install the binary and the app.