add documentation of readonly element in submission-forms.xml

[saschaszott]

The current version of the DSC documentation does not contain any information about the readonly element in submission-forms.xml.

background of this issue: We found a subtle difference in the implementation of the readonly feature of submission-forms.xml between DS7 and DSC7. We would like to get a better understanding of this change. As it is not documented in DSC7 documentation we would also recommend to add a paragraph in the next release of DSC7 PDF documentation.

In DS7 <readonly>readonly</readonly> can be used to declare a submission form element that is readonly.

This commit https://github.com/4Science/DSpace/commit/ae7981b61cb3d8ad14d1a8c5c1fb7451418278eb#diff-e8d9cf5932159917df40848ae0[…]20781a64a49530170eb0eb53d9fR266 changed the behaviour with DSC 2021.01.00.

DSC7 does not support the value “readonly” but expects “all” instead.
This means, that <readonly>all</readonly> is required in DSC7 to get a readonly submission form element.

@abollini added in Slack: DSpace-CRIS is more flexible in this regards than plain DSpace as you can put a field as readonly just in the submission or in the workflow. The readonly xml element is used to tell in which scope or "all" the metadata is readonly.