Restore shield logo as OpenWar brand mark

WarBit storyboards stay distributed through the README as section
illustrations, but the brand mark at the top is back to the shield
(openwar-logo.png). WarBit is the mascot; the shield is the logo.

Removed unused branding/warbit-hero.png (was the WarBit-as-logo
attempt). Mascot work still lives at branding/warbit-story/.

Author: PythonLuvr

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-  <img src="branding/warbit-hero.png" alt="WarBit, the OpenWar mascot" width="260" />
+  <img src="openwar-logo.png" alt="OpenWar" width="160" height="160" />
 </p>
 
 <h1 align="center">OpenWar</h1>