Add Code of Conduct guidelines

CONDUCT.md

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+This document provides community guidelines for a safe, respectful, productive,
+and collaborative place for any person who is willing to contribute to the Opal
+community. It applies to all “collaborative space”, which is 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.