Add Code of Conduct (issue 575620043 by address@hidden)

[janek . lilypond]

Reviewers: pkx166h,

Message:
It seems I cannot create issues on sourceforge despite having an
account...
> Please enter a valid tracker issue number
> (or enter nothing to create a new issue): 
> Error code 403
> Failed URL was
https://sourceforge.net/rest/p/testlilyissues/issues//new

James, can you help?

Description:
>From Mike Solomon, originally discussed at
https://github.com/lilypond/lilypond/pull/8:

> Adds the Contributor Convent Code of Conduct to LilyPond. If we
implemented a
> code of conduct, it would only make sense if we had an enforcement
committee.
> In most organizations, enforcement committee members are exemplary of
the code
> itself.
> 
> The list of adopters is quite robust [1], and I think it has been used
with a lot
> of success. I really hope we can adopt it.
> 
> I would vote for @jan-warchol, @lemzwerg and @hanwen to be the members
of the
> first code of conduct committee.

All suggested members of the committee agree to accept this duty. So,
now the
question is for the whole LilyPond community: do you agree to having
this Code
of Conduct and do you accept the nominees to form the committee?

[1] it includes linux kernel and git project itself.

Please review this at https://codereview.appspot.com/575620043/

Affected files (+131, -0 lines):
  A CODE_OF_CONDUCT.md