Add new help page for bonus tiles, bug 721181

There are eight bonus tiles: four flowers and four seasons. The postmodern
theme has blue winter motives in flower group and yellow spring motives in
season group. The smooth theme has yellow symbols in flower group and black
symbols in season group. The identical bonus tiles are matched in postmodern
theme and the bonus tiles with the same symbol color are matched in smooth theme.
https://bugzilla.gnome.org/show_bug.cgi?id=721181