Update README.md

Clarified a bit how horizontal and vertical slides relate.

README.md

@@ -15,7 +15,7 @@ reveal.js comes with a broad range of features including [nested slides](https:/
 
 ### Markup
 
-Markup heirarchy needs to be ``<div class="reveal"> <div class="slides"> <section>`` where the ``<section>`` represents one slide and can be repeated indefinitely. If you place multiple ``<section>``'s inside of another ``<section>`` they will be shown as vertical slides. For example:
+Markup heirarchy needs to be ``<div class="reveal"> <div class="slides"> <section>`` where the ``<section>`` represents one slide and can be repeated indefinitely. If you place multiple ``<section>``'s inside of another ``<section>`` they will be shown as vertical slides. The first of the vertical slides is the "root" of the others (at the top), and it will be included in the horizontal sequence. For example:
 
 ```html
 <div class="reveal">