add readme text for sse keep alive

services/sse/README.md

@@ -12,3 +12,8 @@ Log services like the `userlog`, `clientlog` and `sse` are responsible for compo
 ## Subscribing
 
 Clients can subscribe to the `/sse` endpoint to be informed by the server when an event happens. The `sse` endpoint will respect language changes of the user without needing to reconnect. Note that SSE has a limitation of six open connections per browser which can be reached if one has opened various tabs of the Web UI pointing to the same Infinite Scale instance.
+
+## Keep SSE Connections Alive
+
+Some intermediate proxies drop connections after an idle time with no activity. If this is the case, configure the `SSE_KEEPALIVE_INTERVAL` envvar. This will send periodic SSE comments to keep connections open.
+