MAINT: Added Code of Conduct to .github folder
https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/

Github treats this file special in a few places. I think
https://github.com/pandas-dev/pandas-governance/blob/master/code-of-conduct.md
should always be considered the "official" home. Perhaps we could add
a note to the one added here about that.

[ci-skip]

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
