Create CITATION.md
Addresses #24036 by adding a `CITATION.md` file to the repository.

I copied the citation instructions from here: https://pandas.io/about/citing.html
and added a section for the published software on Zenodo. Following [this recommendation](https://github.com/sherpa/sherpa/pull/634#issuecomment-553668211), I did this for the latest released version (v1.0.1) with a note for the user to go fetch the citation from Zenodo for the version they are actually using.

Tagging @TomAugspurger and @jreback since you were active in the linked issue.

I still recommend to update the citation request on the pandas website directly (https://pandas.io/about/citing.html), as well as on the Scipy website (https://www.scipy.org/citing.html#pandas), on https://pandas.pydata.org/ and maybe provide a `<package>.__citation__` variable as suggested [here](https://github.com/pandas-dev/pandas/issues/24036#issuecomment-585672595).
