Add CODE_OF_CONDUCT.md

This is essentially 1:1 with Rubocop's Code of Conduct, which itself uses Ruby's
Code of Conduct as a base. It's minimal and easy to understand, which mirrors
the ethos of the Sorcery library itself.

CODE_OF_CONDUCT.md

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+# The Sorcery Community Code of Conduct
+
+This document provides a few simple community guidelines for a safe, respectful,
+productive, and collaborative place for any person who is willing to contribute
+to the Sorcery community. It applies to all "collaborative spaces", which are
+defined as community communications channels (such as mailing lists, submitted
+patches, commit comments, etc.).
+
+* Participants will be tolerant of opposing views.
+* Participants must ensure that their language and actions are free of personal
+  attacks and disparaging personal remarks.
+* When interpreting the words and actions of others, participants should always
+  assume good intentions.
+* Behaviour which can be reasonably considered harassment will not be tolerated.