add note on -i default being changed

ChangeLog

@@ -48,6 +48,13 @@
     sometimes getting in the way (for example if you try to ping a lot of
     hosts).
 
+  * INCOMPATIBLE CHANGE: default interval (-i) changed from 25ms to 10ms
+
+    The default minimum interval between ping probes has been changed from
+    25ms to 10ms. The reason is that 25ms is very high, considering today's
+    fast networks: it generates at most 31 kbps of traffic (for IPv4 and
+    default payload size).
+
   * (feature) Unified 'fping' and 'fping6' into one binary (#80)
   * (feature) Long option names for all options
   * (feature) --enable-ipv6 is now default