Authorise the bot from the following URL.
https://discordapp.com/oauth2/authorize?client_id=418329413860458496&scope=bot&permissions=53484608
Then add a line to the topic of the channel you want notifications:
vc-notification: [CHANNEL]
Specify a space-delimited list of channel names to monitor.
vc-notification: General
When someone joins the Voice Channel General, the bot posts a message to the text channel.