document code generation

DEVELOPMENT.md

@@ -191,6 +191,14 @@ the Rust code used to represent different Python objects and their methods. The
 core implementation of what a Python object is can be found in
 `vm/src/pyobjectrc.rs`.
 
+### Code generation
+
+There are some code generations involved in building RustPython:
+
+- some part of the AST code is generated from `vm/src/stdlib/ast/gen.rs` to `compiler/ast/src/ast_gen.rs`.
+- the `__doc__` attributes are generated by the 
+  [__doc__](https://github.com/RustPython/__doc__) project which is then included as the `rustpython-doc` crate.
+
 ## Questions
 
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