Clarify the project's use of the Typelevel CoC.

In the past I assumed it was obvious that the project required participants to abide by the Typelevel Code of Conduct.

This commit clarifies and makes this explicit through the README. It also names me (Erik) and Typelevel as the points of contact for issues, and provides a link to our GUIDE.md.

Review by @tixxit, @larsrh, and anyone else who is interested to weigh-in.