chain utility note

src/layers/basic.jl

@@ -12,6 +12,7 @@ on a given input.
     m(x) == m[2](m[1](x))
 
 `Chain` also supports indexing and slicing, e.g. `m[2]` or `m[1:end-1]`.
+`m[1:3](x)` will calculate the output of the first three layers.
 """
 type Chain
   layers::Vector{Any}