Consider creating a view mode policy

[Asbjoedt]

Based off Carls findings during his development, consider creating a new file format policy for ODS which applies the chapter 4.16.11 “manifest:preferred-view-mode”

**4.16.11 manifest:preferred-view-mode**
The manifest:preferred-view-mode attribute specifies a preference on how the author of the document would like the document to be presented upon the document being opened. This attribute is only applicable to the root file entry with the manifest:full-path [4.16.4](https://docs.oasis-open.org/office/OpenDocument/v1.3/os/part2-packages/OpenDocument-v1.3-os-part2-packages.html#attribute-manifest_full-path) attribute value of "/".

The defined values for the manifest:preferred-view-mode attribute are:

* edit: The author's preference is to open the document as an editable document.
* presentation-slide-show: The author's preference is to open the document as presentation slide show. 
* read-only: The author's preference is to open the document as a read-only document. 
* A namespaced token value: Specifies an implementation-defined view. 

Preferred view modes are not necessarily generally applicable to all media types. The default preferred view mode is implementation-defined. The behavior for cases where the manifest:preferred-view-mode attribute is absent is implementation-defined.

The requirement could be a MAY or SHOULD along the lines of:

The file MAY/SHOULD have the attribute value "read-only" in the manifest.