todo-txt: restore caveats

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/todo-txt.rb

@@ -12,4 +12,10 @@ class TodoTxt < Formula
     prefix.install 'todo.cfg' # Default config file
     (prefix+'etc/bash_completion.d').install 'todo_completion'
   end
+
+  def caveats; <<-EOS.undent
+    To configure, copy the default config to your HOME and edit it:
+      cp #{prefix}/todo.cfg ~/.todo.cfg
+    EOS
+  end
 end