message: Document that OUT_DIR is set during compilation for packages with build scripts
diff: @@ -49,6 +49,8 @@ let version = env!("CARGO_PKG_VERSION");
  * `CARGO_PKG_NAME` - The name of your package.
  * `CARGO_PKG_DESCRIPTION` - The description of your package.
  * `CARGO_PKG_HOMEPAGE` - The home page of your package.
  +* `OUT_DIR` - If the package has a build script, this is set to the folder where the build
  + script should place its output. See below for more information.
  # Environment variables Cargo sets for build scripts
  