Lost requirement for reading order

[mattgarrish]

In the steps for processing default values, the document that links to the manifest is added to the reading order if there isn't a reading order.

It appears we lost the justification for this when we took the web publications section out, however, as these two statements used to be part of the specification:

[The primary entry page] is an [HTML] resource that is returned when accessing the Web Publication's address, and MUST be included in either the default reading order or the resource list.

The default reading order MAY be omitted when it only consists of the primary entry page. When the default reading order is absent, user agents MUST include an entry for the primary entry page when compiling the canonical manifest.

The current specification says to look at the discovery section to find the justification for adding the linking page to the reading order, but without those statements we have a logic hole.

To make the specification consistent again, we should incorporate these back into the current text.

(It would also make it unnecessary for audiobooks to have to check if document.url is in the unique resources, as this should be checked in the main algorithm.)