Add code of conduct

[agahkarakuzu]

Adding code of conduct based on:

• https://github.com/ME-ICA/tedana/edit/master/CODE_OF_CONDUCT.md
• https://www.contributor-covenant.org/version/1/4/code-of-conduct.html

See #306.

[coveralls]

Coverage increased (+0.01%) to 43.76% when pulling 3886ab2 on coc into 520a9bb on master.

[agahkarakuzu]

@mathieuboudreau accessing from mobile, I may have deleted your comment by mistake.

These are templates, I will edit content.

[mathieuboudreau]

Great contribution @agahkarakuzu ! We might want to re-discuss the templates after several months to see if it fits our needs or if they need to be further customised, but I think this works as a great step forward in the right direction!