chromeos: Add READMEs for ash/dbus and c/b/chromeos/dbus.

Add README.md files in //ash/dbus and
//chrome/browser/chromeos/dbus.

Bug: 843392
Change-Id: I31181b72aabf60f7c09feda7e8081078215ad472
Reviewed-on: https://chromium-review.googlesource.com/1182951
Commit-Queue: Dan Erat <derat@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584738}

ash/dbus/README.md

+Under multi-process ash (mash), these D-Bus services will be owned by the ash
+process. See `//ash/README.md` for details on mash.
+
+Please see [Chrome OS D-Bus Usage in Chrome] for information about adding D-Bus
+services.
+
+[Chrome OS D-Bus Usage in Chrome]: https://chromium.googlesource.com/chromiumos/docs/+/master/dbus_in_chrome.md

chrome/browser/chromeos/dbus/README.md

+This directory contains implementations of D-Bus services that are owned by
+Chrome (because they depend on functionality located within `//chrome`).
+
+Please see [Chrome OS D-Bus Usage in Chrome] for information about adding D-Bus
+services.
+
+[Chrome OS D-Bus Usage in Chrome]: https://chromium.googlesource.com/chromiumos/docs/+/master/dbus_in_chrome.md

chromeos/dbus/services/README.md

-Please see [Chrome OS D-Bus Usage in Chrome] for information about implementing
-new D-Bus services within Chrome.
+This directory contains shared code used to implement D-Bus services.
+
+Please see [Chrome OS D-Bus Usage in Chrome] for information about adding D-Bus
+services.
 
 [Chrome OS D-Bus Usage in Chrome]: https://chromium.googlesource.com/chromiumos/docs/+/master/dbus_in_chrome.md