add note about custom plugins

templates/zshrc.zsh-template

@@ -23,6 +23,7 @@ ZSH_THEME="robbyrussell"
 # COMPLETION_WAITING_DOTS="true"
 
 # Which plugins would you like to load? (plugins can be found in ~/.oh-my-zsh/plugins/*)
+# Custom plugins may be added to ~/.oh-my-zsh/custom/plugins/
 # Example format: plugins=(rails git textmate ruby lighthouse)
 plugins=(git)