Add ebraille reading system section

[mattgarrish]

Here's a first stab at reading system requirements. I went through the epub reading systems spec and listed out all the differences I could find in terms of support.

I'm less sure about the section on web-hosted publications, but leaving it in for now.

One question is do we want to mandate support for PDF for tactile images, should it be a recommendation, or do we need to mention it at all?

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[mattgarrish]

what about a reading system that offloads PDF support to another app?

That's not how EPUB would define support, but that's not really an issue here. We just need to define that as an acceptable behaviour, so maybe I'll move that bullet into a new subsection (it's technically an addition to EPUB's requirements whereas all the rest are removing features).

[mattgarrish]

Rather than add new subsection, I just took the first two bullets and made them separate paragraphs after the list. The question I can't answer is whether there are technical issues that would prevent a pdf from opening in another app, but that's the sort of feedback we'll have to wait on implementers for.

I also added another bullet about not supporting non-xhtml documents in the spine. Not supporting manifest fallbacks doesn't answer what to do if someone were to put another format in the spine. I've basically left it to the reading system to either reject such a publication outright or to ignore any spine items that reference foreign resources.

[mattgarrish]

I noticed I had put a couple of reading system restrictions in the authoring sections, so I moved the one about not resolving path absolute url strings to the new section and removed the redundant one about not supporting scripting and form submissions.