[a11y] Editorial changes required for creating an ISO/IEC spec for EPUB a11y

Folks,

Since we cannot use the fast-track procedure or PAS procedure, we are
required to follow the drafting rules of ISO/IEC.  (Note: EPUB 3.0
did not follow them, since it was fast-tracked.)

ISO/IEC Directives Part 2: Principles to structure and draft documents
intended to become International Standards, Technical Specifications
or Publicly Available Specifications.

http://www.iso.org/sites/directives/2016/part2/index.xhtml#_idTextAnchor055

I think that there are two significant changes.

First, we have to provide Foreword, Introduction, and Scope as
required by Directives Part 2.  This requires some work, although
we can use some text from "1. Overview" in EPUB A11Y 1.0.

Second, we have to use SHALL, MAY, and other modal verbs as required
by Directives Part 2.  Most notably, all MUSTs have to be replaced by
SHALLs.

Regards,
Makoto