Add code of conduct suggested by github

[mossroy]

This is the code of conduct that GitHub suggested when I started adding a similar file.
I find it suitable for us, but we can adapt it if we want (or replace it by another one we would prefer)

Fixes #61

[Popolechien]

Question: how different is it from the CoC (itself derived from the standard Contributor Covenant) found at https://github.com/kiwix/kiwix-android/blob/develop/CODE_OF_CONDUCT.md ?

[mossroy]

@Popolechien : interesting, I was not aware this file existed for kiwix-android!
It seems to be the same source but an older version (from contributor-covenant 1.4, where this PR is from 2.0)
The email is also slightly different : [email protected] on kiwix-android, where I had simply put [email protected]

[Jaifroid]

The code of conduct looks good, with just one possible edit noted. Do you want to include a link to it from the README in this PR?

[Jaifroid]

Oops, sorry, I thought I was reviewing this on Kiwix JS. Of course the PR can't include the links to it in that case.

[Popolechien]

Would suggest using [email protected] as email address

[mossroy]

Would suggest using [email protected] as email address

Done

[mossroy]

@kelson42 and @Popolechien : do you want more changes on this Code Of Conduct?

[Popolechien]

@mossroy Fine by me, but @kelson42 is the boss here.

[mossroy]

@kelson42 : can we merge this PR?
It would allow us to copy this file in kiwix-js too