docs: clarify compatibility parser positioning

Author: tassis

README.md

@@ -5,4 +5,16 @@
 
 KDL grammar for [tree-sitter](https://github.com/tree-sitter/tree-sitter)
 
-Adapted from the [official spec](https://github.com/kdl-org/kdl/blob/main/SPEC.md)
+Adapted from the [official KDL specification](https://kdl.dev/spec/)
+
+## Compatibility
+
+`tree-sitter-kdl` currently aims to be a permissive compatibility grammar for
+editor and tooling use-cases.
+
+It accepts commonly-used syntax from both KDL v1 and KDL v2 so existing `.kdl`
+files continue to parse well in generic Tree-sitter integrations.
+
+This package should not be treated as a strict KDL version validator. In
+particular, it may successfully parse documents that mix KDL v1 and KDL v2
+literal forms.