update branches description

readme.md

@@ -9,7 +9,14 @@ It thus should be able to run on the architecture and ISA the players have built
 The `main` branch contains a minimal implementation of the game.
 
 The following branches contains variations, allowing comparisons of performances, visuals, gameplay, ...
-- `res-2`: 256x192 screen resolution, 17x17 tile
+- `res-2`:
+  - `256x192` screen resolution
+  - `17x17` tile resolution
+- `res-2-prerendered`:
+  - `256x192` screen resolution
+  - `17x17` tile resolution
+  - initial screen prerendered
+  - no sprite rotation
 
 ## Build script