add comment to build.rs that it isn't released

Author: msrd0

build.rs

@@ -1,3 +1,8 @@
+//! WARNING - This file is not part of the release
+//!
+//! We only use this file to emit `cfg(nightly)` which
+//! is only necessary for benchmarks.
+
 fn main() {
 	let version_meta = rustc_version::version_meta().unwrap();
 	// Enable "nightly" cfg if the current compiler is nightly.