Add changes file for ticket 10842.

This is a bugfix on 0.2.2.26-beta, because 6b83b3b made directory
authorities remove themselves from the list of directory authorities to
upload to, but didn't suppress the warning in case they're the only
directory authority in the network.

changes/bug10842

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Suppress a warning that votes and signatures cannot be uploaded to
+      other directory authorities if there's only one directory authority
+      in the network. Bugfix on 0.2.2.26-beta. Resolves ticket 10842.