Add README for Chromium sensor backend modernization on Windows.

This document introduces the proposed design changes for modernizing
the Chromium sensor backend on Windows allowing it to use the new
Windows.Devices.Sensor WinRT APIs instead of the deprecated ISensor Win32 API.

This README will not cover the context or justification for the
modernization work as it is already captured by this document:
https://docs.google.com/document/d/1Vr5xXOu4v-H6ydeCJed1bU_WR9kBYm3ANV-GB-JPZuw/edit?usp=sharing

The new README file also reads more like a future looking document
(contains "will do this..." and "proposed") since it not implemented yet.

Change-Id: Ibcfeb9020bd930d50ab9898ce0ee145c9e645e1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1664111
Commit-Queue: Wenjie Shi <wensh@microsoft.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672220}
4 files changed