It's nothing new but very few people know about this feature on lollipop.
Go to System > Settings > Smart Lock and select Trusted Face. Go through the on screen instructions and voila!
Beware, face matching is less secure than a password, pattern or four digit PIN and if anyone looks like you, they can potentially unlock your phone.
***Edited to add labels***
I hate to bump this. but a photograph could be used
Like If I was nickle back and had my phone locked with face look. Then this Gif could unlock the phone.
Even a printed high rez colour still with the right lighting could.
Face Should not be used at all!