[Documentation] Add several READMEs

Missing README files for directories of which I'm an OWNER.

Change-Id: I48a3bd92f0c156901edfca48c341d0634bc415fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2297521Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#788520}

components/cookie_config/README.md

+Implements a cross-platform net::CookieCryptoDelegate that implements encryption
+and decryption of the cookie store if required on a per-platform basis.

content/browser/device/README.md

+Implementation of //content API's device_service.h.

services/device/README.md

+Provides cross-platform public interfaces and per-platform backing
+implementations of various features related to the user's physical device.