Add API intro

engines/fullgame/API.md

 # Draft for the complete game engine API
 
-This is just a draft.
+The goal of this small library is to provide a simple framework for creating 2D games.  
+The map is a 2D grid, with tiles (floor, obstacles...) and entities (boxes to push, monsters, items...).
+Inputs can be prompted or read from a file, and a simplistic events manager is given.  
+The game can then be played in a console, with ASCII drawings. To create an actual graphic interface we will probably use Easytracker.
 
 
 ## Including the library and common stuff