message: Mention the stderr file in the documentation.
diff: @@ -58,6 +58,9 @@ cargo:libdir=/path/to/foo/lib
  cargo:include=/path/to/foo/include
  ```
  +On the other hand, lines printed to stderr are written to a file like
  +`target/debug/build/<pkg>/stderr` but are not interpreted by cargo.
  +
  There are a few special keys that Cargo recognizes, some affecting how the
  crate is built:
  