Add optional regex constraints

examples/regex/README.md

@@ -0,0 +1,9 @@
+# Regex example
+
+Regex constraints are optional. Run this example with the `regex` feature enabled:
+
+```sh
+cargo run -q -p decodal --features regex -- examples/regex/main.dcdl
+```
+
+Without the feature, regex validation returns an unsupported feature diagnostic.