added topic about ETH address book in Thunderbird

mail/how_to_use_email_with_thunderbird.markdown

@@ -52,3 +52,9 @@ You can check your settings and finish the configuration.
 [[!img /media/how_to_use_email_with_thunderbird/t8.png alt="Thunderbird (8 of 8)"]]
 
 You will then be asked for the password to read mail.  This finishes the configuration and you can start reading your mail.
+
+Adding ETH address book
+----------------------
+
+You can add the ETH address book in Thunderbird.
+Just follow [these instructions](https://unlimited.ethz.ch/spaces/itkb/pages/283251869/Thunderbird+address+book+ETH+Active+Directory).