Well, it's difficult to reference the schema when it is in a diffrent Git repo.

We could write something like:

```shell
wget https://raw.github.com/yast/yast-installation-control/master/control/control.rng
xmllint --noout --relaxng control.rng <XML_FILE>
rm control.rng
```

But that will not be correct when you want to check against a different product (i.e. a different Git branch, there can be some changes in the future, so there might a problem with openSUSE vs SLE validation...)

IMHO we can write only something like
```
Make sure to use the control.rng file matching the target
product code base (i.e. validate on the installated product
or use the correct Git branch when checking out the schema).
```