add link for an important reference in recon script

mu_map/recon/osem.py

@@ -270,6 +270,6 @@ if __name__ == "__main__":
 
     recon_image = recon_image[
         :, :, ::-1
-    ]  # STIR creates reconstructions flipped on the x-axes, this is reverted here
+    ]  # STIR creates reconstructions flipped on the x-axes (https://sourceforge.net/p/stir/mailman/message/36938120/), this is reverted here
     recon = Interfile(recon_header, recon_image)
     write_interfile(args.out, recon)