Chainfire released a newer version of his root package earlier today, dubbed as SuperSU 2.56. This one avoids mounting your system partition, meaning no modification are made to it.
This helps you keep your device ready for OTA updates, which require that no modifications are made to system partition.
Check our full guide on how to use new SuperSU 2.56 to root Marshmallow running Nexus devices here.
Download the latest SuperSU 2.56 root package below.
In order to install/flash SuperSU 2.56, copy it to your device first. And then flash a custom kernel of permissive type that supports root access, and then boot into TWRP recovery and use the Install button to flash the SuperSU 2.56 to achieve root access.