update readme with crane sprocket image

README.md

@@ -5,7 +5,7 @@
 Folder | Description
 ------ | -----------
 [bearings](parts/bearings#readme) | Ball bearings based around part 168
-[crane-sprocket](parts/crane-sprocket#readme) | Sprockets and idlers for large track system
+[<img src="parts//crane-sprocket/images//meccano-large-track-sprocket.png" width="100">](parts/crane-sprocket#readme) | Sprockets and idlers for large track system
 [<img src="parts/custom-parts/differential/images/bevel1.jpg" width="100">](parts/custom-parts#readme) | Adapters for non Meccano parts and new spins on existing Meccano parts
 [<img src="parts/dredger-bucket/images/rect.png" width="100">](parts/dredger-bucket#readme) | Two different forms of dredger bucket
 [<img src="parts/electrical/images/520.png" width="100">](parts/electrical#readme) | Elektrikit and Electronics sets parts