Add a Code of Conduct

[wadells]

The teleport repo got a code of conduct here:

gravitational/teleport@9d27061

and gravity got the same here:

gravitational/gravity#925

We should include our code of conduct in the teleconsole repo. Although teleconsole isn't currently high activity, it is still a relatively popular repo overall.

Extra credit: A CONTRIBUTING.md file with information about filing bugs, security disclosures, et al.

[wadells]

Filing this as a bug to help Kafi learn our engineering tools & process during August 2020 engineering onboarding 😄

[wadells]

More info here: https://docs.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors