add comment (#454)

Author: st--

src/transform/transform.jl

@@ -5,6 +5,9 @@ Abstract type defining a transformation of the input.
 """
 abstract type Transform end
 
+# We introduce our own _map for Transform so that we can work around
+# https://github.com/FluxML/Zygote.jl/issues/646 and define our own pullback
+# (see zygoterules.jl)
 Base.map(t::Transform, x::AbstractVector) = _map(t, x)
 _map(t::Transform, x::AbstractVector) = t.(x)