Add note about compatibility with IMSC

[nigelmegitt]

See also w3c/imsc#622.

Add a new informative appendix about "Compatibility with other TTML-based specifications" and a subsection on IMSC.

Broadly, a conformant DAPT document is also a conformant IMSC Text document, as long as it meets the constraints of both specifications. Where processing by an IMSC processor is desirable, the IMSC Text profile designator can be included in ttp:contentProfiles as an additional value to the DAPT profile designator.

It may be worth mentioning that:

• adding layout and styling to a DAPT document may be a useful way to produce a subtitle or caption file
• the computed values of xml:lang and daptm:srcLang may be useful to derive appropriate values for itts:forcedDisplay
• in some applications it may be better to map a single DAPT document into multiple IMSC Text documents, each intended for a specific single language. This can be done by pruning the content with an unwanted language, for example.
• in some applications it may be better to process the text content of a DAPT document to block it out into presented lines or blocks of lines of text in distinct ISDs, for readability.

[nigelmegitt]

I've made a hopefully reasonable attempt at this in #333 also mapped in w3c/imsc#625.