docs: gnome-maps: mention some quirks

hosts/common/programs/gnome-maps.nix

@@ -1,6 +1,15 @@
 # SUPPORT:
 # - irc: #gnome-maps on irc.gimp.org
 # - Matrix: #gnome-maps:gnome.org  (unclear if bridged to IRC)
+#
+# INTEGRATIONS:
+# - uses https://graphhopper.com for routing
+#   - <https://github.com/graphhopper/graphhopper>  (not packaged for Nix)
+# - uses https://tile.openstreetmap.org for tiles
+# - uses https://overpass-api.de for ... ?
+# TIPS:
+# - use "Northwest" instead of "NW", and "Street" instead of "St", etc.
+#   otherwise, it might not find your destination!
 { pkgs, ... }:
 {
   sane.programs."gnome.gnome-maps" = {