update readme with new URI scopes

Readme.md

@@ -76,8 +76,10 @@ For now there are three types of such rules:
 In case an URI is encountered and there is no applicable rule, it will be rejected
 (note that this means you'll have to explicitly allow `https://` for links!)
 
-For now there are two (useful) scopes: `map` applies to tiled map links 
-(i.e. `exitUrl`), and `website` applies to weblinks (i.e. `openWebsite`).
+There are currently four possible scopes: `map` applies to tiled map links 
+(i.e. `exitUrl`), `website` to `openWebsite`, `audio` to `playAudio`, and
+`bbb` to Big Blue Button rooms (though that last one may be changed again,
+depending on the bbb deployment in use).
 
 
 ## Output