CONTRIBUTING.md: clarify that Bash 4+ optimizations are allowed

CONTRIBUTING.md

@@ -9,6 +9,9 @@ General guidance
   That's because that's the version shipped with MacOS.
   (They didn't upgrade past it and switched to Zsh because later versions
   are covered by GPLv3 which has additional restrictions unacceptable for Apple.)
+  
+  You can still add performance optimizations and such that take advantage of newer Bash features
+  as long as there is a fallback execution route for Bash 3.
 
 * Be extra careful when submitting logic specific for the Apple Silicon platform