feat(cli): expose server negotiation policy

README.md

@@ -202,6 +202,23 @@ irtt-server \
     --idle-timeout 30s
 ```
 
+Two optional controls restrict what the server will provide a session, and both
+are off unless asked for:
+
+```sh
+irtt-server \
+    --bind 192.0.2.10:2112 \
+    --timestamp-allowance single \
+    --no-dscp
+```
+
+`--timestamp-allowance` takes `dual` (the default, honoring every requested
+placement), `single`, which provides at most one timestamp and answers a request
+for both receive and send timestamps with the midpoint, or `none`, which provides
+no timestamps. The requested clock source is never changed. `--no-dscp`
+negotiates any requested DSCP to zero, so clients are told their echo replies
+will be unmarked, and they are sent unmarked.
+
 To require authentication, pass the shared key both sides use:
 
 ```sh