pep8: add README (#7248)

plugins/pep8/README.md

+# pep8 plugin
+
+This plugin adds completion for [pep8](https://pep8.readthedocs.io/en/release-1.7.x/#), a tool to check your Python code against some of the style conventions in [PEP 8](http://www.python.org/dev/peps/pep-0008/).
+
+To use it, add pep8 to the plugins array of your zshrc file:
+```
+plugins=(... pep8)
+```