exo: Add some more info to the README.

Change-Id: I34548f56ef60ca5ef1fa3cba934ea86f3797677e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2376941Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Chloe Pelling <cpelling@google.com>
Cr-Commit-Position: refs/heads/master@{#802419}

components/exo/README

-This directory contains an implementation of a display server on top of
-the Aura Shell.

components/exo/README.md

+Exo implements a display server on top of the Aura Shell. It uses the
+[Wayland protocol](https://wayland.freedesktop.org/docs/html/)
+to communicate with clients. For a general introduction to Wayland see
+https://wayland-book.com/.
+
+Current clients of Exo include:
+
+* ARC++ (Android apps on Chrome OS)
+* [Chromecast](https://chromium.googlesource.com/chromium/src/+/master/chromecast/README.md)
+* Crostini (Linux apps on Chrome OS)
+* [Lacros](https://chromium.googlesource.com/chromium/src/+/master/docs/lacros.md)
+* PluginVM
+
+In addition to the core Wayland protocol, Exo supports a number of protocol
+extensions. Some are third-party; see
+[//third_party/wayland-protocols/README.chromium](https://chromium.googlesource.com/chromium/src/+/master/third_party/wayland-protocols/README.chromium).
+Others are Chromium-specific.
+
+A few noteworthy extensions (this list is not at all exhaustive):
+
+* zaura_shell
+  * A Chromium-specific protocol used by all Exo clients. See
+    [//components/exo/wayland/protocol/aura-shell.xml](wayland/protocol/aura-shell.xml)
+    and [//components/exo/wayland/zaura_shell.h](wayland/zaura_shell.h)
+* zcr_remote_shell
+  * A Chromium-specific protocol used exclusively by ARC++. See
+    [//components/exo/wayland/zcr_remote_shell.h](wayland/zcr_remote_shell.h) and
+    [//components/exo/client_controlled_shell_surface.h](client_controlled_shell_surface.h)
+* zwp_fullscreen_shell
+  * A third-party protocol, used in Chromium only by Chromecast. See
+    [//components/exo/wayland/zwp_fullscreen_shell.h](wayland/zwp_fullscreen_shell.h)