note bug with thunerbird attachments, fixed

doc/usage.rst

@@ -141,6 +141,10 @@ examples of known working configurations.
 
     monkeysign --mua "thunderbird -compose to=%(to)s,subject=%(subject)s,body=%(body)s,attachment=%(attach)s" [...]
 
+  .. note:: Thunerbird fails to respect the attachment parameter in
+            versions before 52.1.1, see :bts:`837771` for more
+            details.
+
 * Mutt::
 
     monkeysign --mua "mutt -a %(attach)s -s %(subject)s -i %(body)s %(to)s" [...]