Add note about Thunderbird patches

wiki/src/blueprint/monthly_report/report_2019_05.mdwn

@@ -49,6 +49,11 @@ Code
 - We spent lots of time dealing with the Firefox add-ons fiasco,
   also known as "armagadd-on 2.0". [[!tails_ticket 16694]]
 
+- Finally, most of our patches improving the security of Thunderbird's
+  email configuration wizard ([[!tails_ticket 6156]]) were merged and
+  will be part of Thunderbird 68, that is: not just in Tails, but on all
+  for all Thunderbird users on any operating system!
+
 Documentation and website
 =========================