docs(readme): note that GDS runs standalone, BMM optional but recommended

Clarifies install expectations for users only interested in game dev:
the BMM module isn't required, but pairs well with GDS and ships
updates more frequently.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sjennings

README.md

@@ -65,7 +65,9 @@ BMGD is installed as a module during BMad Method setup:
 npx bmad-method install
 ```
 
-Select **Game Dev Studio** from the modules list.
+Select **Game Dev Studio** from the modules list. 
+
+*Just a heads-up: if you’re only using Game Dev Studio (GDS), you don’t need to install the main BMM module. GDS runs on its own, but BMM gets updated more often, so you might want to use its workflows—they work together nicely!*
 
 ## Quick Start