Fix plugin documentation

Mention that reveal.js will wait for the Promise of a plugin's init()
function only when the plugin is loaded non-async. The init functions
of plugins that are loaded as async dependencies are called after
reveal.js has dispatched the 'ready' event.

README.md

@@ -1268,6 +1268,7 @@ Reveal.addEventListener( 'ready', () => console.log( 'Three seconds later...' )
 Reveal.initialize();
 ```
 
+For plugins that are loaded as [dependencies](#dependencies), reveal.js will wait for the fullfillment of their init Promise only for the *non-async* plugins. 
 If the init method does _not_ return a Promise, the plugin is considered ready right away and will not hold up the reveal.js startup sequence.
 
 ### Retrieving Plugins