message: Added documentation for LINKER environment variable
  This commit amends the documentation on environment variables to
  reflect the proposed LINKER variable.
diff: @@ -123,6 +123,8 @@ let out_dir = env::var("OUT_DIR").unwrap();
  * `RUSTC`, `RUSTDOC` - the compiler and documentation generator that Cargo has
  resolved to use, passed to the build script so it might
  use it as well.
  +* `LINKER` - The linker that rust has resolved to use for the current target,
  + passed to the build script so it might use it as well.
  [links]: reference/build-scripts.html#the-links-manifest-key
  [profile]: reference/manifest.html#the-profile-sections
  