Updated CONTRIBUTING.md with install instructions for autopep8

Summary: autopep8 is not intuitive to install for devs not familiar with it and/or pip.

Test Plan: Installed using this method.

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1585