Add advice on writing on behalf of someone else

[abibroom]

(TL;DR don't.) This is the most common cause of messages getting trapped by the 'address has a postcode in it' filter: "I'm writing on behalf of my mum who is one of your constituents", they've used mum's postcode to get the correct rep, and then filled in their own details including postcode in an address field where it shouldn't go.

There is of course no guarantee that people will read the FAQs, but maybe this will help a little bit.