If you have earned at least one Play! Point this year, then you will get a code for a Shiny HA (Unnerve) Mewtwo! This is the only Shiny HA Mewtwo that is legitimately obtainable so far (I'm pretty sure)
Details on Play! Points are here: http://www.pokemon.com/us/play-pokemon/about/play-points/
Source: http://www.pokemon.com/us/strategy/unnerve-your-foes-with-mewtwo/ [And serebii...]
I will try my best to put notifications here as they come for major events