Remove rebasing from contributing guide
If I'm not wrong, now GitHub squashes commits automatically when merging pull requests [1]. In the pandas documentation there is a section about combining commits [2], that doesn't seem relevant anymore if that is being used.

Would it make sense to delete that section, and may be add a comment about using the title of the pull request for the "right" commit message that will end up in the git history? 

1. https://help.github.com/articles/about-pull-request-merges/#squash-and-merge-your-pull-request-commits
2. https://pandas.pydata.org/pandas-docs/stable/contributing.html#combining-commits