Google's newest version of Android has finally begun branching out to phones outside of the Pixel lineup, and the Nexus 5, one of the first such devices, has a build of Android 8.0 (Oreo) that's far from suitable for daily use at this time. The highly unstable and unoptimized ROM is currently without almost any features, but the fact that a build of Oreo boots up on a 32-bit device at all, let alone with working Wi-Fi, is still promising. Other core features to make this ROM a daily driver may come in time, but developer Santhosh M over on XDA Developers boards makes it very clear to prospective downloaders that this build is not for daily use, and they do not want to receive ETA requests for getting parts of the OS working; that's something that will likely come with time, as more and more people build out to different devices with Oreo.
In order to flash this ROM, you'll want to have your Nexus 5's bootloader unlocked, and have a custom recovery on board. Doing that on a Nexus device is pretty simple, if you haven't already; go into your Developer settings and check "OEM Unlock" or the equivalent option, then root the device with your method of choice, and use a tool like the TWRP app or Flashify, or simply flash your preferred recovery image by hooking your rooted and bootloader-unlocked phone up to a computer with ADB. Once you've done that, it's as simple as downloading the files you want to flash, booting up your recovery, and flashing them. You can dirty-flash the files on top of your current system, which is incredibly risky and may result in a hard brick, or you can wipe your system, cache, and Dalvik cache partitions before flashing. Regardless, it's probably wise to take a backup before flashing as there is absolutely no guarantee that the phone will boot. If you'd like to flash Google Apps, a package is available, but you should be aware that using it will likely decrease the stability of this already highly unreliable ROM.
As with any early build of a new version of Android, users can expect things to get better with time. The Nexus 5 is an aging device and runs on a 32-bit processor, so it may never be able to use some features of the OS to their fullest extent. This build is meant mainly for enthusiasts of the highest order, who simply want to try the newest version of Android, and developers who want to see how the ROM runs on the Nexus 5 and help build it in the future.