message: Document the rustc-cdylib-link-arg key
diff: @@ -62,6 +62,7 @@ cargo:rustc-link-lib=static=foo
  cargo:rustc-link-search=native=/path/to/foo
  cargo:rustc-cfg=foo
  cargo:rustc-env=FOO=bar
  +cargo:rustc-cdylib-link-arg=-Wl,-soname,libfoo.so.1.2.3
  # arbitrary user-defined metadata
  cargo:root=/path/to/foo
  cargo:libdir=/path/to/foo/lib
  @@ -93,6 +94,9 @@ crate is built:
  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.
  +* `rustc-cdylib-link-arg=FLAG` is a flag passed to the compiler as
  + `-C link-arg=FLAG` when building a `cdylib`. Its usage is highly platform
  + specific. It is useful to set the shared library version or the runtime-path.
  * `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
  