doc: Document new app boilerplate requirement
doc: Document new app boilerplate requirement

Document that the boilerplate code included in all applications must
now also include

cmake_minimum_required(VERSION 3.8.2)

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

zephyrproject-rtos/zephyr,kraj/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,punitvara/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,kraj/zephyr,kraj/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr