Prepare CR draft ready for publication

[nigelmegitt]

• Add Exit Criteria
• Create Implementation Report page
• Reference Implementation Report in SOTD
• Add acknowledgements
• Change maturity stage to CRS (CR Snapshot)

[nigelmegitt]

I propose to set the Exit Criteria to be:

For this specification to exit the CR stage,
at least 2 independent implementations of every feature defined in this specification
but not already present in [TTML2] need to be documented in the implementation report.
The Working Group does not require that implementations are publicly available but encourages them to be so.

[nigelmegitt]

I have created the Implementation Report page.

[nigelmegitt]

Pull request prepared for this at #269.

[nigelmegitt]

I checked the TTWG Charter's Success Criteria which says:

When considering suitability to advance any feature beyond Candidate Recommendation, at least two independent factors of verification for each normative requirement MUST be demonstrated, which may come, as relevant for that requirement, from any of:

• Presentation implementation
• Content-producing implementation
• Validating implementation

I think my proposed CR Exit Criteria are compatible with that, without needing a change to be more specific.

[himorin]

I'm somehow curious about which DAPT feature is in TTML or not, in the implementation report.
It is clear that all tt/ttp/tta are TTML and daptm are DAPT, but I suppose we may be better to have some more text than every feature defined in this specification but not already present in [TTML2] in the report, since the report is to be checked by non-familiar colleagues from TTML world..

[nigelmegitt]

@himorin the DAPT features are those defined as feature extensions at https://www.w3.org/TR/dapt/#extensions

[himorin]

@nigelmegitt Ahhh, I totally forgot about that section... Thank you for reminding!