Include a style guide for contributors

[diracdeltas]

We should have consistent coding styles.

[diracdeltas]

We still aren't really PEP8 compliant. Should this be a goal?

I use https://github.com/klen/python-mode with

au FileType python setlocal tabstop=8 expandtab shiftwidth=4 softtabstop=4        
let g:pymode_lint = 1                                       
let g:pymode_lint_checker = "pyflakes,pep8"

[garrettr]

This is still an issue. We've had several pull requests come through recently that have fixed a number of PEP8 violations, which has helped. I think we should re-orient this issue to not only recommend a consistent coding style, but to enforce it. Therefore, the requirements for this issue should be:

• Make coding style adherence a requirement for contribution, and document the requirement in our contribution guidelines (docs/develop.md).
• Enforce this requirement via our CI pipeline. For example, Let's Encrypt checks code style as part of their Travis automated tests.