Merge pull request #2378 from mbotsch/fix-plugin-documentation

Fix plugin documentation regarding init-Promises

README.md

@@ -1292,6 +1292,7 @@ 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