Document the use of SocketIO in README.md

README.md

@@ -64,6 +64,27 @@ enough.
 Flamenco Manager has a SwaggerUI interface at http://localhost:8080/api/swagger-ui/
 
 
+## SocketIO
+
+[SocketIO v2](https://socket.io/docs/v2/) is used for sending updates from
+Flamenco Manager to the web frontend. Version 2 of the protocol was chosen,
+because that has a mature Go server implementation readily available.
+
+SocketIO messages have an *event name* and *room name*.
+
+- **Web interface clients** send messages to the server with just an *event
+  name*. These are received in handlers set up by
+  `internal/manager/webupdates/webupdates.go`, function
+  `registerSIOEventHandlers()`.
+- **Manager** typically sends to all clients in a specific *room*. Which client
+  has joined which room is determined by the Manager as well. By default every
+  client joins the "job updates" and "chat" rooms. This is done in the
+  `OnConnection` handler defined in `registerSIOEventHandlers()`.
+- Received messages (regardless of by whom) are handled based only on their
+  *event name*. The *room name* only determines *which* client receives those
+  messages.
+
+
 ## Database
 
 Flamenco Manager and Worker use SQLite as database, and Gorm as