Added missing breaking change about history files

Author: kmvanbrunt

CHANGELOG.md

@@ -31,6 +31,8 @@
     * Changed default behavior of `runcmds_plus_hooks()` to not stop when Ctrl-C is pressed and instead
       run the next command in its list.
     * Removed `cmd2.Cmd.quit_on_sigint` flag, which when `True`, quit the application when Ctrl-C was pressed at the prompt.
+    * The history bug fix resulted in a change to the `cmd2.HistoryItem` class. Therefore, persistent history
+      files created with versions older than 2.0.0 are not compatible.
 * Enhancements
     * Added support for custom tab completion and up-arrow input history to `cmd2.Cmd2.read_input`.
       See [read_input.py](https://github.com/python-cmd2/cmd2/blob/master/examples/read_input.py)