Document changes in who.ini post-OpenID removal

After #2058 (OpenID support removal), there are some changes needed in the `who.ini` file to remove the OpenID related config options. If not removed, they cause an exception on first load:

```
ImportError: <module 'ckan.lib.authenticator' from '/usr/lib/ckan/master/src/ckan/ckan/lib/authenticator.py'> has no 'OpenIDAuthenticator' attribute
```

Changes needed are basically [these](https://github.com/ckan/ckan/pull/2058/files#diff-2), and should be listed in the CHANGELOG