Added leaflet-tracksymbol to plugin page.

[ghost]

I would be very happy if you add my leaflet plugin link to the plugin page.

[IvanSanchez]

Looks good! A couple of comments, though:

• If your plugin creates a new kind of L.Marker, it should define a L.Marker.TrackMarker class, as per the Leaflet coding conventions
• It would help a lot if you could provide a live example of the plugin in action, or at least a screenshot