Install Android File Transfer and back up important data on phone including photos, videos, sms messages, music, etc. If you use Google to back up your contacts, double check they're all synced. This guide makes no claim that your app settings/logins will be preserved, even if you follow the backup steps described below. OK enough disclaimers, LET'S START HACKING!
You'll need to download the command line tools fastboot and adb, both for communicating with your phone while it's plugged into your computer. Various rooting packages online contain these programs but the safest place to download them is just to grab the whole Android ADK for your OS and locate these tools inside the platform tools directory. You may want to copy fastboot and adb to another folder closer to your home folder for easier command line navigation later (like your desktop).
Go back one screen (to the main settings menu) and select Developer options. Check the box to enable USB debugging.
In Settings -> Security, check the box to allow installation of apps from Unknown sources.
If you haven't already, use system update to make sure your phone is running Android 4.4.2.
You already remembered to separately back up your important files like photos and videos, right? The next step will wipe all the personal data from your device, so now's your last chance to back up. When the backup is finished, power down your phone and unplug it from USB.
Press and hold the Power and Volume down buttons until you see the bootloader menu. The bottom line of text should say your phone is locked.
Now plug your phone into your computer with a microUSB cable.
In Terminal type:
Press the Volume up button to select Yes. Then press the Power button to confirm. After erasing, the bottom line of text should now say unlocked. Congrats, you unlocked the bootloader! Press the power button again to reboot your phone.
In Settings -> About phone, you will have to re-enable developer options by tapping the build number repeatedly, then in Developer options turn USB debugging back on. In Settings -> Security, re-check Unknown sources to allow apps from unknown sources.
All that modding was for one purpose: to swap out the NfcNci.apk file, which is accomplished by downloading and running this updater for NfcNci.apk for cyanogenmod11/android 4.4.2. Use Android File Transfer to drag this zip file onto your phone.
Then select go back until you see the reboot system now option, and select it to restart your phone.
Congrats, now your phone will poll NFC on the lock screen! Test it out by holding a tag to your phone when the screen is on but still locked-- you should hear the quiet noise that indicates a tag has been read!
Now it's time to set up the custom actions that will unlock the phone when the correct NFC tag is near the phone.