Nikon has issued Firmware Update C: version 1.10 for the D810 DSLR, effective October 29, 2015.
This update makes mostly minor tweaks to images captured in Live View mode, but it also adds an External Recording Control option to the HDMI item in the Setup Menu. This feature allows the camera controls to start and stop recording to 3rd-party HDMI recorders that support the Atoms Open Protocol.
Read the full list of changes/ bug fixes and download the firmware update here.