Auto merge of #3758 - mbrubeck:docs, r=alexcrichton

Document that OUT_DIR is set during compilation for packages with build scripts

None

Author: bors

src/doc/environment-variables.md

@@ -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