Auto-respons on pm

Hi there
I am looking for a feature that many BNC's provide.
As soon as someone PM's me I want to start the conversation with a 
certain string.

Let's say I want everyone that PM's me to see the text:
"Hi there! I will get to you as soon as I can, please type your full 
question here so that I may help you faster"

Not just when I am away but all the time, so people know this.
Is there a addon or maybe just a function that does this?

Best regards
XeroXer