Merge pull request #245 from dandv/patch-1

Document vertical centering breaking backcompat

README.md

@@ -93,6 +93,8 @@ Reveal.initialize({
 });
 ```
 
+Note that the new default vertical centering option will break compatibility with slides that were using transitions with backgrounds (`cube` and `page`). To restore the previous behavior, set `center` to `false`.
+
 ### Dependencies
 
 Reveal.js doesn't _rely_ on any third party scripts to work but a few optional libraries are included by default. These libraries are loaded as dependencies in the order they appear, for example: