Add vendor flash instructions for bullhead (Nexus 5X)

Summary

On first install on a Nexus 5X, i was greated by a "Vendor mismatch" and "ui stopped" alerts. Solving this require flashing the right vendor version. This should be added to the documentation.

Possible fixes

The vendor version required (8.1 OPM7.181205.001) can be found here. This link come from this XDA thread

Basically, all there is to do is rebooting into bootloader and flash with fastboot flash vendor vendor-bullhead-opm7.181205.001.img