Now that we have a Code of Conduct we need to advertise it. Github is standardizing CODE_OF_CONDUCT.md in the root of the repo as the place to put the CoC text; following this seems useful both because it is a reasonable convention where no other exists and because all our code is mirrored to Github.
Per discussion in this task and elsewhere, MediaWiki extensions and skins seem like the reasonable target for automated adding of the file; there are repos which are only used internally or where the file could cause problems, and while there are plenty of repos which should probably have the file (e.g. PHP libraries), except for extensions/skins there doesn't seem to be an easy way to list them.
See also: