doc: release notes: RISC-V release notes for 2.0
doc: release notes: RISC-V release notes for 2.0

Adds 2.0 release notes for the RISC-V architecture.

Signed-off-by: Nathaniel Graff <e7e2d91f1deb937cded6d6fc8e8292f24ce08299@sifive.com>

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