Split contributors/article.sgml into several files.

Note to committers: Lists of core, committers, and contributors are
split into contrib.core.sgml, contrib.committers.sgml,
and contrib.additional.sgml respectively.

Translators: Please use the split files as entities in the
localized article.sgml instead of translating all of the
original files. See ja_JP.eucJP version for example.

Not objected to by: -doc