How to install Android N ( Version 7) on Nexus 5

You can install Android N (AOSP) build on your Nexus 5 and use the functionality such as multi windows support, quick notification reply on your Nexus 5 as well. 

Before going ahead, I would remind you that by following this method will void your device warranty/guaranty. There are chances that your device may get bricked (unusable). So make sure you follow the steps correctly and do not stop the process in between.

Requirements:

• Working android SDK platform tools ( fastboot and dab)
• Updated USB driver for your device installed on computer.
• USB debugging enabled on device. Check this for how to enable USB debugging on mobile.
• Download recovery from TWRP 
• Download latest Android N (AOSP) image from xda-dev forum, you can choose any other image shared on forum.

Unlocking Bootloader

• Once you have enabled USB debugging on device and connected device to computer execute below command on command prompt or terminal
• adb reboot bootloader
• Once your device restarted, check if the device is in fastboot by executing below command
• fastboot devices
• Above command should show your device serial number. If the device serial number is displayed then execute below command
• fastboot oem unlock
• A disclaimer will display on the device that must be accepted. Use the volume keys to cycle through the options. Pressing the power button should confirm your selection.
• Reboot the device from menu if device does not rebooted automatically. Now the bootloader should be unlocked. After reboot if you see the unlocked icon on the bottom of boot screen menu. 

Installing TWRP recovery

• Connect your device to computer via USB.
• On command prompt or terminal execute below command
• adb reboot bootloader
• Check if the device is on fastboot mode like above. 
• Make sure the recovery image is in the directory from where you are executing commands.
• If the device is in fastboot mode, flash recovery entering below command
• fastboot flash recovery recovery_image_file_name.img
• Once the flash completes successfully, make sure you reboot the device. 
• To check if the recovery completed successfully, reboot device in recovery mode, you should see the TWRP recovery menu.
• In order to manually go to recovery mode -> Powered down the device. Now hold the Volume Down and Volume Up buttons. Then press and hold the Power button until a boot menu appears. Navigate using the volume keys and select RECOVERY using the Power key.

Installing Android N(AOSP)

• Copy the Android image zip to SD card, you can use the below command to do the same after connecting your device to computer via USB. ( USB debugging is still on)
• adb push android_image_file_name.zip /sdcard/ 
• Reboot your device to recovery mode

In TWRP there is an option to backup. You should create a backup before installing custom ROM so that you can go back if anything goes wrong.

Once you have taken a backup:

• Select Wipe and then Factory Reset.
• Select Install.
• Navigate to /sdcard and select the android_image_file_name.zip package.
• Follow the on-screen notices to install the package.

Once installation has finished, return to the main menu and select Reboot, then System. The device will now boot into Android N.

You can follow the xda-dev forum for troubleshooting if required.