Add a Code of Conduct

[mossroy]

I would suggest to add a "Code of Conduct" for Kiwix.

We can write our own, or get inspired by existing ones (I don't think we need anything specific to Kiwix), like :

• https://socratesuk.org/code_of_conduct.html
• https://mixitconf.org/en/codeofconduct

[kelson42]

@mossroy Agree and it should probably be the same for the whole project/organisation. I have an old ticket to even syncrhonise this file on each code repo, see kiwix/container-images#152

[mossroy]

Good idea!