Suggestion to the Instructions page about backports.org's gpg key

[Gálos András <galosa@netinform.hu>]

It may be useful to tell people on the Instructions page how to add the 
backports.org archive's gpg key to apt's keyring to avoid the aptitude 
warnings in etch.

With something like this:

If you are using etch and you want apt to verify the downloaded 
backports you can import backports.org archive's key into apt:

gpg --keyserver hkp://subkeys.pgp.net --recv-keys 16BA136C
gpg --export | apt-key add -


Thank you,

Regards,
Andras Galos