Create a section on options for customisation

[nigelmegitt]

As per Action-460, and recorded in these minutes, add a section to TTML2 explaining the options for customisation, including as a minimum:

• deliberate non-compliance of presentation processors with the specification;
• local setting of implementation dependent values such as default value for tts:color etc.;
• use of parameters and conditions to control styling for using.

This will also serve as a placeholder for any other mechanisms that we introduce for customisation.

[skynavga]

Labeling this for ttml.next and closing. If someone wishes to post a PR on this for inclusion in the upcoming WR, then please do so and reopen this issue.

[nigelmegitt]

@skynavga I don't recall discussing this issue and agreeing to move to ttml.next - I would prefer to leave issues open and not done, and mop this up in an activity later on. It may be higher on someone else's priority list than yours.

[skynavga]

Moving off editor's WR list due to lack of PR.

[nigelmegitt]

Adding back on to editor's WR list now that there is a PR.