[css-text] Add tagging guidance as an appendix to CSS3 Text

[fantasai]

Some guidance in http://fantasai.inkedblade.net/style/specs/text-justify-i18n/#guide which was transcribed and updated into i18n's http://w3c.github.io/i18n-drafts/articles/typography/justification.en.html probably doesn't belong there. It might fit reasonably well as an informative appendix to CSS3 Text, however.