Update changelog

ChangeLog.txt

@@ -6,6 +6,11 @@ Broker:
 - Fix std* files not being redirected when daemonising, when built with
   assertions removed. Closes #2708.
 
+Client library:
+- Use CLOCK_BOOTTIME when available, to keep track of time. This solves the
+  problem of the client OS sleeping and the client hence not being able to
+  calculate the actual time for keepalive purposes. Closes #2760.
+
 Clients:
 - Fix incorrect topic-alias property value in mosquitto_sub json output.