Document how to use externally supplied iwyu (e.g. windows)

[peakschris]

We are excited to try bazel_iwyu, but as a Windows shop, we understand it is not supported OOTB at present. It would be really helpful if you could document how to use this on Windows. We are happy to provide the IWYU binary, but are not sure how to configure bazel_iwyu to use it. We are currently getting this error on Windows:

ERROR: An error occurred during the fetch of repository 'iwyu_prebuilt_pkg':
   Traceback (most recent call last):
        File "D:/udu/b/fsve7lhs/external/com_github_storypku_bazel_iwyu/bazel/prebuilt_pkg.bzl", line 9, column 13, in _prebuilt_pkg_impl
                fail("Unsupported arch: {}".format(arch))
Error in fail: Unsupported arch: x86_64

Thank you!