I added document explaining ninjalog collection.
This document explains what type of data is collected and how the data will be used.

Bug: 900161, b/120645342
Change-Id: If47517aaa7a5510e4a634b5ccc0e4127d014dcb9
Reviewed-on: https://chromium-review.googlesource.com/c/1381912
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>