Add a changes file for the heartbeat code

changes/heartbeat

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+  o Minor features
+    - Add a new 'Heartbeat' log message type to periodically log a
+      message describing Tor's status at level Notice.  This feature
+      is meant for operators who log at notice, adn want to make sure
+      that their Tor server is still working.  Implementation by
+      George Kadianakis.    
+