TTML2 Editing Process

I have documented the (new) TTML2 Editing Process [1]. This process adopts
the standard github pull-request mechanism; therefore, it provides the
advantages of fine-grained review of issue-based merges.

The only variation to what has been followed with IMSC1 is that I have
retained the commit-then-review (CTR) process that has been used for TTML1
and TTML2 to date.

[1] https://github.com/w3c/ttml2/blob/gh-pages/EDITING.md