Server protocol: Advertize relay servers in welcome message

server-protocol.md

@@ -86,6 +86,20 @@ and handle them accordingly, if present:
   future version of the protocol requires a rate-limiting CAPTCHA ticket or
   other authorization record, the server can send `error` (explaining the
   requirement) if it does not see this ticket arrive before the `bind`.
+* `relays`: An advertizement list of relay servers. It is a JSON list of which each
+  entry may look like this:
+    ```json
+    {
+      "url": "tcp:myrelay.example.org:12345",
+      "country": "Italy",
+      "continent": "EU",
+    }
+    ```
+  The only mandatory key is `url`, al others are optional information to help the client
+  choose an appropriate one. Further keys may be added in the future. Clients must not
+  expect the protocol to be `tcp` (expect websockets support in the future). Clients
+  should make a preselection of viable relay servers (which may include entries from other
+  sources as well), and randomly select one or two.
 
 The first thing each client sends to the server, immediately after receiving
 the welcome message, is a `bind` message. This specifies the