The Nexus devices are always the first to receive the latest and greatest software from Google. Even older devices like the Nexus 5 generally end up getting the updates. Applying the update is not always the easiest for those of us who like to root and rom our devices. Accepting an OTA update is not an option for those of you with a custom recovery installed to your device. In order to get the update you would need to manually flash the image, which also replaces your custom recovery with the stock recovery.
If you would rather update to Android 6.0 on your Nexus 5 as pain free as possible you will probably want to flash a custom rom. CyanogenMod 13 is now available as an official build. This will bring your device up to Android 6.0. The rom is further optimized for better battery performance and speed. You also get some customization options. You will need to be bootloader unlocked with custom recovery installed to use this. Head to the link below for the download.
via Cyanogen