Add changes file for #25610

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/ticket25610

+  o Major feature (directory authority, modularization):
+    - The directory authority subsystem has been modularized. The code is now
+      located in src/or/dirauth/ which is compiled in by default. To disable the
+      module, the configure option --disable-module-dirauth has been added.
+      Closes ticket 25610;