Add local-only build scripts example in check-cfg docs

Author: Urgau

src/doc/src/reference/build-scripts.md

@@ -292,6 +292,32 @@ if foo_bar_condition {
 }
 ```
 
+##### Example of local-only `build.rs`/build script
+
+Build scripts can impose costs on downstream users, and crate authors who wish to avoid
+this can use "local-only" build scripts, which are active locally but not packaged in the
+distributed package. Completly removing the cost from downstream users. 
+
+The way to achieved this, is by excluding the `build.rs` in the `Cargo.toml` with the
+`exclude` key:
+
+```diff
+  [package]
+  name = "foo"
+  version = "0.1.0"
++ exclude = ["build.rs"]
+```
+
+*`build.rs`:*
+```rust
+fn main() {
+    // Warning: build.rs is not published to crates.io.
+
+    println!("cargo::rerun-if-changed=build.rs");
+    println!("cargo::rustc-check-cfg=cfg(foo)");
+}
+```
+
 For a more complete example see in the [build script examples][build-script-examples] page
 the [conditional compilation][conditional-compilation-example] example.