Add a hint in shortcut_properties.py on how to make it work from a clean depot_tools install

R=grt@chromium.org

Change-Id: Ibcc042cddf089adb42aafb5d98fcb71888306ae3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210022Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770990}

chrome/installer/tools/shortcut_properties.py

@@ -6,6 +6,9 @@
 
 This is required to confirm correctness of properties that aren't readily
 available in Windows UI.
+
+If you run into "ImportError: No module named pywintypes":
+Run this script with vpython instead.
 """
 
 import optparse