Tweak changes file: call this a major feature.

changes/bug21530

-  o Minor bugfixes (relay):
-    - If we are a relay where ExitRelay is auto and we have not specified
-      an exit policy, whether through ExitPolicy lines or ReducedExitPolicy,
-      emulate the behavior of ExitRelay as 0. Previously, we allowed exit
-      traffic if ExitRelay is auto and logged a warning message. Fixes bug
-      21530; bugfix on 0.3.5.1-alpha. Patch by Neel Chauhan.
-
+  o Major features (relay):
+    - Relays no longer run as exits by default. If the "ExitRelay" option is
+      auto (or unset), and no exit policy is specified with ExitPolicy or
+      ReducedExitPolicy, we now treat ExitRelay as 0. Previously in this
+      case, we allowed exit
+      traffic and logged a warning message. Closes ticket
+      21530. Patch by Neel Chauhan.