Add recommendation about typeset -g

Code-Style-Guide.md

@@ -32,6 +32,10 @@ Use existing variables whenever possible:
 * Good: `${PWD}` `${USER}`
 * Bad: `$(pwd)` `$(whoami)`
 
+When declaring a global variable, be explicit and do it using `typeset -g`. This avoids warnings when `WARN_CREATE_GLOBAL` is set:
+* Good: `if (( ! ${+VAR} )) typeset -g VAR=1`
+* Bad: `: ${VAR=1}`
+
 Flow logic
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