Add code of conduct

[Hoverbear]

Use the contributor covenant for several reasons:

• It focuses us, as contributors, pledging to make a space safe for participation.
• It highlights how to positively participate in the community first. Describing unacceptable behavior after, creating a positive framing.
• It clearly outlines consequences and domains of affect.

Note: @grahamc , could you please ensure the coc-grahamc@determinate.systems and coc-ana@determinate.systems email points to your and my emails appropriately? We want to have two contacts and have them clearly listed who the recipient is so that if a contact needs to occur the person is aware who will receive the message, and have a second option in case their report relates to the person who would receive the email otherwise.

[linear[bot]]

DS-307 Create CODE_OF_CONDUCT.md

[grahamc]

Accounts exist. Looks good to me.