Add a README.md to the google_update dir.

BUG=none
TBR=grt@chromium.org

Change-Id: Iff6eb54c3cff2bd162d799c9b1e977345da641f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270300Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782943}

google_update/README.md

+This directory contains the IDL file used by Google Chrome to interact with
+Google Update. Note that this file should not be used directly by other Chromium
+forks that interact with their own [Omaha](https://github.com/google/omaha)
+forks.
\ No newline at end of file