Document availability of realtime channels and how to test for it (#79)

* Document availability of realtime channels and how to test for it

* Update src-docs/spec/joinRealtimeChannel.md

Co-authored-by: Sebastian Klähn <[email protected]>

---------

Co-authored-by: Sebastian Klähn <[email protected]>

Author: Simon-Laux

src-docs/spec/joinRealtimeChannel.md

@@ -18,6 +18,15 @@ Per-app realtime channels are:
 Calling `joinRealtimeChannel` a second time without leaving the prior one
 will throw an error.
 
+## Check if the API is available
+
+This API is experimental and may not be implemented by every messenger yet.
+Check if it is available with `window.webxdc.joinRealtimeChannel !== undefined`
+ 
+For Delta Chat, you need at least version 1.46 to enable the feature
+in the settings under `Settings -> Advanced -> Experimental Features -> Realtime Webxdc Channels`.
+Without enabling the feature, the API is available but not working.
+
 ## `realtimeChannel.setListener((data) => {})` 
 
 Start listening on the realtime channel using the specified callback.