Add text about Dragonfly to explaining-bsd-article

[Joel Dahl]

Dragonfly has recently reached 1.0, and is seriously entering the scene.
/usr/share/doc/en_US.ISO8859-1/articles/explaining-bsd/article.html
covers FreeBSD, NetBSD, OpenBSD, BSD/OS and Mac OS X very briefly, how
about adding some information about Dragonfly to it? I could easily
write something for someone to review. Any objections? :)