add a build.rs file to the stable-only syntex example

README.md

@@ -129,6 +129,28 @@ fn main() {
 }
 ```
 
+`build.rs`
+
+```rust
+extern crate syntex;
+extern crate serde_codegen;
+
+use std::env;
+use std::path::Path;
+
+pub fn main() {
+    let out_dir = env::var_os("OUT_DIR").unwrap();
+
+    let src = Path::new("src/main.rs.in");
+    let dst = Path::new(&out_dir).join("main.rs");
+
+    let mut registry = syntex::Registry::new();
+
+    serde_codegen::register(&mut registry);
+    registry.expand("", &src, &dst).unwrap();
+}
+```
+
 This also produces:
 
 ```