Move and edit comment, add link

Author: Gregliest

docs/src/models/advanced.md

@@ -12,10 +12,10 @@ struct CustomModel
 end
 
 function (m::CustomModel)(x)
-  return m.chain(x) + x
-
-  # You can put arbitrary code here, but note that everything here will be differentiated.
+  # Arbitrary code can go here, but note that everything will be differentiated.
   # Zygote does not allow some operations, like mutating arrays.
+
+  return m.chain(x) + x
 end
 
 # Call @functor to allow for training. Described below in more detail.
@@ -30,6 +30,8 @@ model = CustomModel(chain)
 model(rand(10))
 ```
 
+For an intro to Flux and automatic differentiation, see this [tutorial](https://fluxml.ai/tutorials/2020/09/15/deep-learning-flux.html)
+
 ## Customising Parameter Collection for a Model
 
 Taking reference from our example `Affine` layer from the [basics](basics.md#Building-Layers-1).