proto: give a rough protocol overview as part of the docs

proto/protocol.xml

@@ -30,6 +30,29 @@
   </copyright>
 
   <documentation>
+    <chapter title="Protocol overview">
+      The ei protocol has three components: objects, requests and events. It is
+      designed to be connect two processes over a UNIX socket - an ei client and
+      an EIS implementation (typically a Wayland compositor). The protocol
+      is asynchronous and object-based.
+
+      Whenever a message (request or event) is sent, that message carries an
+      identifier for the object. The type of an object is defined by its interface -
+      the interfaces are detailed here in this protocol. Thus, when a message for an
+      object arrives, the client or EIS implementation can invoke the corresponding
+      function on the object.
+
+      An object has exactly one interface but there may be multiple objects with
+      the same interface (e.g. multiple devices all use the ei_device interface).
+
+      Requests are messages sent from an ei client to an EIS implementation, events
+      are messages sent from the EIS implementation to the client. This is the same
+      nomenclature that the Wayland protocol uses.
+
+      The ei protocol is modelled closely after the Wayland protocol, but it is not
+      binary compatible.
+    </chapter>
+
     <chapter title="Wire format">
     Protocol wire format: `[sender-id, length, opcode, ...]`
     Where: