Issue-309 "Image profile fails WCAG 1.2"

I've added the following note to Issue-309:

This was discussed at the HTML accessibility media sub-group on 14 July, minutes at [1], with Nigel and Pierre present.

[1] http://www.w3.org/2014/07/14-html-a11y-media-minutes.html

The conclusion was that though alt text may be provided, it is not essential, and it is not a requirement to permit some form of alternate text to images in the specification, as long as there is a known mechanism for providing alternate text based media with appropriate metadata. An analogy was made to different language translations of the same content.

Follow-up actions:
* add a note to describe this scenario (where an accessible text version of image profile content is provided) and how it should be handled, advising content providers what they should do;
* describe what mechanism external to IMSC documents should be present to facilitate this.

Kind regards,

Nigel