Update Design Doc: `level` tool

Design-Doc%3A-%60level%60-tool.md

@@ -25,7 +25,10 @@ The string property `reskit-layer` set on a Tile Layer indicates which layer sho
 Tile layers without `reskit-layer` set are ignored by the `level` tool.
 
 ## Collision Map
-The boolean property `reskit-collision` set on a Tile Layer to `true` indicates which layer should be used for building the tile collision map. Any non-zero item placed indicates a tile which the player is not permitted to pass through. How you implement this is up to your game engine - reskit will simply save the collision map in a `.bin` file as an artifact.
+The boolean property `reskit-collision` set on a Tile Layer to `true` indicates which layer should be used for building the tile collision map. Any non-zero item placed indicates a tile which the player is not permitted to pass through. How you implement this is up to your game engine - reskit will simply save the collision map in a `.bin` file as an artifact (`collision.bin`).
+
+### Format of the Collision Map
+The collision map is just an array of 8-bit unsigned integers laid out with the same tile dimensions as your visible layers. 0 is a free area, while 1 is a collision area.
 
 ## Entity-Component System
 Reskit assumes a best-practice entity-component engine design and the objects defined in your .tmx file effectively serve as the entities. By adding a property `reskit-component[x]` to each object, where x is the index of the component in an array, you can attach various components to the target object/entity. Each component may have various properties you choose to define, which can be formatted as Tiled property `reskit-component[x].y`, where x is the index of the component and y is the property name for that component. If multiple object layers are defined, reskit will collapse all of them into a single layer.