doc(engine): explain how visitors are generated

engine/DEV.md

@@ -42,6 +42,12 @@ To show the file nicely formated, use: `dune describe pp lib/types.ml` (or `dune
 
 You can also use `dune utop` and then `#show_type Hax_engine.Types.SOME_TYPE` and `#show_constructor Hax_engine.Types.SOME_CONSTRUCTOR`.
 
+### Visitors
+The module `Ast_visitors` provides a `Make` functor that takes a feature set and outputs a module that defines the `map`, `mapreduce` and `reduce` classes.
+
+Those visitors are generated by `./utils/generate_visitors`. 
+To see the implementation of the `Ast_visitors` module, run `dune describe pp lib/ast_visitors.ml`.
+
 ## Debugging the phases
 You can enable a debug mode that prints a Rustish AST at each phase,
 that you can browse interactively along with the actual AST.