mklog: Add --append option to auto add generate ChangeLog to patch file

This tiny patch add --append option to mklog.py that support add generated
change-log to the corresponding patch file. With this option there is no need
to manually copy the generated change-log to the patch file. e.g.:

Run `mklog.py --append /path/to/this/patch` will add the generated change-log
to the right place of the /path/to/this/patch file.

contrib/ChangeLog:

	* mklog.py: Add --append option.

Signed-off-by: Lehua Ding <lehua.ding@rivai.ai>