So, you love the beastly hardware included in the Samsung Galaxy S6, but you aren't loving TouchWiz. You want to go with an alternative flavor of Android, but you don't want to void your phone's warranty. With the way Samsung locks down the S6 you've basically been left without a solution - that is, until now. On the XDA developer forums a clever dude calling himself idler1984 has posted an APK and his instructions for using his tool called PingPong Root, which obtains root status on a Samsung Galaxy S6 without tripping up Samsung's prohibitive KNOX policing software. The process circumvents KNOX's detection by providing a way to obtain root without the user being required to use Samsung's ODIN software for flashing a custom recovery program onto the device. In following his method, the phone's warranty is not modified, so it's rational to assume that Samsung would accordingly honor your phone's warranty term should you need to make a claim sometime in the future.
Not all variants of the Galaxy S6 are yet supported for this mod; as of this writing, the posted list of successfully tested configurations consists of only a couple dozen ROM versions running on G920 variants. The mod's author says that other arm64 devices, such as the Galaxy S6 Edge, LG G Flex 2, and HTC One (M9) are among those that we can expect will be rootable using PingPong Root in the future, though no timeframe is mentioned - he says they'll "look into them soon :)" (smiley face and all).
PingPong Root is still very much in a beta phase, and as is typically the case with betas, one can encounter bugs and other issues with the software. Initial reports coming into the PingPong Root thread on XDA are mentioning difficulty in getting programs to recognize root status if SuperSU isn't the root manager installed. Exercise wise judgment and caution if you decide to take the plunge, though you can rest assured that many people have already successfully rooted their S6 using the tool. If you are curious enough to root your Galaxy S6 but don't want to go the PingPong route, we posted instructions last month for a method that uses Samsung's ODIN - which probably trip the warranty and void your warranty, but it is a more officially supported way to go.