Point to prosody image with http-lua built in

Author: jaywink

README.md

@@ -101,6 +101,8 @@ The prosody image needs to have the Lua module `http` installed. Install it with
 luarocks install http
 ``` 
 
+A Dockerfile [also exists](https://github.com/matrix-org/docker-jitsi-meet/commit/e096b4ed266b624b98d27d25c9dfb0fc47c9f2aa) with this built in.
+
 ## License
 
 Apache 2.0