DOC: Mention black and PEP8 in pandas style guide

[dsaxton]

black and PEP8 are pretty important elements of code style in pandas but they aren't mentioned in the style guide, so it may make sense to include them.

https://pandas.pydata.org/pandas-docs/stable/development/code_style.html

[alimcmaster1]

We mention them in the contributing guide?

(Maybe a link would be helpful)

https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#python-pep8-black

[gfyoung]

The purpose of the style guide to express particular style points we follow. black and pep8 are just tools we use to enforce general "stylistic correctness".

I agree with @alimcmaster1 that linking is probably a sensible thing to do.