ARROW-4006: Add CODE_OF_CONDUCT.md
ARROW-4006: Add CODE_OF_CONDUCT.md

Many people are not aware that The ASF has a code of conduct. Having this document in the root directory will increase awareness of the type of professional behavior we expect from members of our community.

Author: Wes McKinney <wesm+git@apache.org>

Closes #3179 from wesm/ARROW-4006 and squashes the following commits:

6e88d8ab <Wes McKinney> Add CODE_OF_CONDUCT.md

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