DOC adding CoC to the repo (scikit-learn#16173)

Author: adrinjalali

Diff for: CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+We are a community based on openness and friendly, didactic, discussions.
+
+We aspire to treat everybody equally, and value their contributions.
+
+Decisions are made based on technical merit and consensus.
+
+Code is not the only way to help the project. Reviewing pull requests,
+answering questions to help others on mailing lists or issues, organizing and
+teaching tutorials, working on the website, improving the documentation, are
+all priceless contributions.
+
+We abide by the principles of openness, respect, and consideration of others of
+the Python Software Foundation: https://www.python.org/psf/codeofconduct/
+