Adding Code of Conduct file (#769)

Summary:
This is pull request was created automatically because we noticed your project was missing a Code of Conduct file.

Code of Conduct files facilitate respectful and constructive communities by establishing expected behaviors for project contributors.

This PR was crafted with love by Facebook's Open Source Team.
Pull Request resolved: https://github.com/facebookresearch/fastText/pull/769

Differential Revision: D14672570

Pulled By: EdouardGrave

fbshipit-source-id: 16aaffe4dd5e7b6db362cf75ca7405d54b041594

CODE_OF_CONDUCT.md

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+# Code of Conduct
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.fb.com/codeofconduct) so that you can understand what actions will and will not be tolerated.