[Retros] Add units: 16th update

[Mu-Tsun Tsai]

Here's a N12Ai problem that uses 20 pieces. Although a 20 piece N12Ai
problem can also be easily obtained by modifying Le Gleuher's N16Ai problem,
but this one reduces the number of officers.

Diagram:
http://www.janko.at/Retros/d.php?ff=6n1/pppppppp/8/8/8/3n4/PPPPPPPP/1N4N1
Stip: Add 12 units.


Mu-Tsun