update branches description

readme.md

@@ -9,6 +9,11 @@ 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-0-prerendered`:
+  - `80x60` screen resolution
+  - `5x5` tile resolution
+  - initial screen prerendered
+  - no sprite rotation
 - `res-2`:
   - `256x192` screen resolution
   - `17x17` tile resolution