[CrOS Network] Add src/chromeos/network README.

Creating a basic README for chromeos/network directory.
See https://bit.ly/chromium-readme-tracker.

Fixed: 1102454
Change-Id: I09d92b50e664447d17d6537fca16e023cdb3c098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2281556
Commit-Queue: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803681}

chromeos/network/README.md

+# src/chromeos/network
+
+This directory contains ChromeOS networking related code. This primarily
+consists of network handler classes that are responsible for communicating
+with [Shill], the Chrome OS network manager daemon. See
+[//chromeos/dbus/shill][1] for shill dbus client code.
+
+[1]: https://source.chromium.org/chromium/chromium/src/+/master:chromeos/dbus/shill/
+[Shill]: https://source.chromium.org/chromiumos/chromiumos/codesearch/+/master:src/platform2/shill/