add to plugins array with more portable += instead of =

.zshrc

@@ -90,7 +90,7 @@ ZSHCUSTOM=${ZSH_CUSTOM}
 # Custom plugins may be added to $ZSH_CUSTOM/plugins
 # Example format: plugins=(rails git textmate ruby lighthouse)
 # Add wisely, as too many plugins slow down shell startup.
-plugins=(
+plugins+=(
   git
   gunstage
 )