message: Document the feature
diff: @@ -51,6 +51,7 @@ All the lines printed to stdout by a build script are written to a file like `ta
  cargo:rustc-link-lib=static=foo
  cargo:rustc-link-search=native=/path/to/foo
  cargo:rustc-cfg=foo
  +cargo:rustc-env=FOO=bar
  # arbitrary user-defined metadata
  cargo:root=/path/to/foo
  cargo:libdir=/path/to/foo/lib
  @@ -73,6 +74,12 @@ crate is built:
  * `rustc-cfg=FEATURE` indicates that the specified feature will be passed as a
  `--cfg` flag to the compiler. This is often useful for performing compile-time
  detection of various features.
  +* `rustc-env=VAR=VALUE` indicates that the specified environment variable
  + will be added to the environment which the compiler is run within.
  + The value can be then retrieved by the `env!` macro in the compiled crate.
  + This is useful for embedding additional metadata in crate's code,
  + such as the hash of Git HEAD or the unique identifier of a continuous
  + integration server.
  * `rerun-if-changed=PATH` is a path to a file or directory which indicates that
  the build script should be re-run if it changes (detected by a more-recent
  last-modified timestamp on the file). Normally build scripts are re-run if
  