docs: Note not to put private keys in /usr/share/ostree
docs: Note not to put private keys in /usr/share/ostree

https://bugzilla.gnome.org/show_bug.cgi?id=757524

gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree