docs: Add nautilus-autorun-software man page

nautilus-autorun-software is the default "x-content/unix-software"
handler for GNOME desktop. So it is not just internal tool and thus
it should have a man page. Let's add a simple man page for it.

It resolves the following downstream bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1725766

docs/meson.build

@@ -1,3 +1,4 @@
 install_man('nautilus.1')
+install_man('nautilus-autorun-software.1')
 
 subdir('reference')

docs/nautilus-autorun-software.1

@@ -0,0 +1,26 @@
+.TH nautilus-autorun-software 1 "3 July 2019"
+
+.SH NAME
+Run Software \- helper tool
+
+.SH SYNOPSIS
+.B nautilus-autorun-software
+.RI "" "mount-uri"
+.br
+
+.SH DESCRIPTION
+.B nautilus-autorun-software
+is a helper tool for the GNOME desktop to start software from media with
+Autostart files.
+.br
+
+It is the default "x-content/unix-software" handler for the GNOME desktop.
+.br
+
+.SH SEE ALSO
+Autostart files are described at
+https://specifications.freedesktop.org/autostart-spec/autostart-spec-latest.html
+
+.SH BUGS
+Bug reports can be found and filed at
+https://gitlab.gnome.org/GNOME/nautilus/issues