Bug#869277: /usr/bin/notify-send: Improve the manpage

[c.buhtz at posteo.jp]

Related to

 - "libnotify-bin: add informations about upstream"
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=869409

 - "libnotify-bin: option -a is not described in notify-send man page"
   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=756151

 - "Missing man page for notify-send"
   https://bugzilla.gnome.org/show_bug.cgi?id=757151

 - "add valuable informations to README"
   https://bugzilla.gnome.org/show_bug.cgi?id=785286

Just to be clear: IMO a Debian maintainer shouldn't waste it's time
with pimp up the bad documentation of some projects. It is up to
upstream!