doc: add a link to the code of conduct

doc: add a link to the code of conduct

GitHub has defined a set of standard meta files to include with projects
hosted on GitHub. According to the GitHub Insights Community page for
go project the only one missing is the code of conduct.

Go has a code of conduct on it's website and we should link to it in the
prescribed `.github/CODE_OF_CONDUCT.md` so that people can find it in
the standard location on GitHub. This would be consistent with the
contribution guidelines that are linked to in `.github/CONTRIBUTING.md`.

Ref: https://help.github.com/articles/adding-a-code-of-conduct-to-your-project/

Fixes #22685

Change-Id: Ie89aa39d3df741d7d6ed2dba5c8ba3d0e0dbf618
Reviewed-on: https://go-review.googlesource.com/77231
Reviewed-by: Gabriel Aszalos <gabriel.aszalos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>