lang: add credit to macro

crates/lang/src/ast.rs

@@ -1,6 +1,8 @@
 use crate::lst_parser::syntax_kind::SyntaxKind::*;
 use crate::SyntaxNode;
 use rowan::Language;
+
+// Heavily modified version of https://github.com/rust-analyzer/rowan/blob/e2d2e93e16c5104b136d0bc738a0d48346922200/examples/s_expressions.rs#L250-L266
 macro_rules! ast_nodes {
     ($($ast:ident, $kind:ident);+) => {
         $(