The Meaning of Life, the Universe, and Everything.
All of the documentation I've been able to find has centered around "models" and "texture" resource packs. However, there are a few add-ons which manipulate your HUD and even the pause screen like this one: http://mcpedl.com/coordinates-unlocker-addon/
That one shows your coordinates in the lower right and, needless to say, it's very useful. However I have no idea how he did it. I can spot the place where he added it to hud_screen.json, but I'm not sure how that modification ended up printing the string "Position: x y z"
Any ideas? Eventually I'd like to write an Add-on that displays Village-center information which would make iron farms a lot easier to build etc. if possible...