MAINTAINERS: add an explicit entry for release notes
MAINTAINERS: add an explicit entry for release notes

Add an entry for the release notes files so that release notes PRs get
tagged as "Release Notes", that should make them easier to manage close
to release time.

Signed-off-by: Fabio Baltieri <1c55e39c05d9ab42261288cf330256b2af6ba179@google.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr