If you are looking to root your Galaxy S6 or even if you have already rooted the S6 you will want to grab the latest version of SuperSU from Chainfire. The latest build 2.49 includes fixes for the Galaxy S6, and according to Chainfire includes some SELinux stuff for newer 5.x firmware. If you are running Lollipop and are rooted you will probably want to head to the link below, grab this version, and flash it in twrp.
via XDA