Add changes file for #28518.

See: https://bugs.torproject.org/28518

changes/bug28518

@@ -0,0 +1,4 @@
+  o Minor features (FreeBSD):
+    - Warn relay operators if the "net.inet.ip.random_id" sysctl (IP ID
+      randomization) is disabled on their relay if it is running on FreeBSD
+      based operating systems. Closes ticket 28518.