doc: addition of a code of conduct
doc: addition of a code of conduct

This change adds a Code of Conduct based on
https://www.contributor-covenant.org/. Additional reporting and
enforcement guidelines will be added at a later date.

Signed-off-by: Thea Aldrich <0b9d1648d01237c866298611809c2d438d4a58da@gmail.com>

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