DOC: link to pandas-cookbook instructions

Some people were confused about how to use this cookbook, so adding a link to the setup instructions.

see: https://github.com/jvns/pandas-cookbook/issues/7
this SO question: https://stackoverflow.com/questions/21868105/problems-with-pandas-tutorial-missing-file-bikes-csv-and-special-characters-in
Merge pull request #6435 from jvns/patch-2

DOC: link to pandas-cookbook instructions