Format for TTML2 Branch Names

When pushing a local branch to the TTML2 repository, please ensure that the
name of the branch takes the following form:

issue-NNNN-<description>

e.g.,

issue-0232-image-style-properties

and not

issue-232-image-style-properties

that is, don't forget the leading zero(es) to pad the issue number to four
digits.

G.