Add code of conduct

[olemb]

https://docs.github.com/en/free-pro-team@latest/github/building-a-strong-community/adding-a-code-of-conduct-to-your-project

[bast]

Yes, this is very important.

[rdoursenaud]

I suggest that we take inspiration from the Python Software Foundation's code of conduct.

I'd also add it as docs/code_of_conduct.rst to keep things consistent with our existing contributing.rst. The /docs folder is fully supported by GitHub as outlined in their documentation.

[bast]

Thanks for this good suggestion! Would you like me to add it?

[rdoursenaud]

@bast Go ahead if you feel like it!

[bast]

I will add it this weekend.