Custom Field on Settings page
You can’t change or add fields to the General Settings page, but for site wide available content Craft has this concept of Globals.

See the documentation on it:
https://craftcms.com/docs/globals

It’s also very easy to manage user permissions for Globals, so you can allow certain user groups (besides admins) to change the meta data, somthing that (currently) isn’t possible for everything in Settings.