doc: release notes: mention Pinnacle 100 board addition
doc: release notes: mention Pinnacle 100 board addition

Add mention of new ARM based board:
Added the Pinnacle 100 Modem Development board.

Signed-off-by: Ryan Erickson <445db09b136432317bdbb8e4308a30651390c2e2@lairdconnect.com>

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