Update README.md

README.md

@@ -109,6 +109,7 @@ The present focus is on dealing with some remaining performance limitations that
 
 There is not presently a high-level interface to which we are yet commiting, but if you want to compute the gradient of a function, take a look at `value_and_pullback!!` / `value_and_gradient!!`.
 They both provide a high-level interface which will let you differentiate things, and their implementation demonstrates how an `rrule!!` / rrule-like function should be used.
+(There are a couple of things that you have to get right when using `rrule!!` / functions returned from `build_rrule`, so it's best to use `value_and_pullback!!`).
 
 *Note:* I have found that using a mixture of `PProf` and the `@profview` functionality from Julia's `VSCode` extension essential when profiling code generated by `Taped.jl`.
 `PProf` provides complete type information on its flame graphs, which is important for figuring out what is getting called, but it doesn't highilght type-instabilities.