Add instructions for background images reveal.js (#4325)

MANUAL.txt

@@ -4384,6 +4384,47 @@ the [Beamer User's Guide] may also be used: `allowdisplaybreaks`,
 `allowframebreaks`, `b`, `c`, `t`, `environment`, `label`, `plain`,
 `shrink`.
 
+Background in reveal.js
+-----------------------
+
+Background images can be added to self-contained reveal.js slideshows.
+
+For the same image on every slide, use the reveal.js configuration 
+option `parallaxBackgroundImage` either in the YAML metadata block 
+or as a command-line variable. You can also set `parallaxBackgroundHorizontal`
+and `parallaxBackgroundVertical` the same way and must also set
+`parallaxBackgroundSize` to have your values take effect.
+
+To set an image for a particular slide, add `{data-background-image="/path/to/image"}`
+to the first slide-level header on the slide (which may even be empty).
+
+In reveal.js's overview mode, the parallaxBackgroundImage will show up 
+only on the first slide. 
+
+Other background settings also work on individual slides, including
+`data-background-size`, `data-background-repeat`, `data-background-color`, 
+`data-transition`, and `data-transition-speed`.
+
+See the [reveal.js documentation](https://github.com/hakimel/reveal.js#slide-backgrounds)
+for more details.
+
+For example:
+
+```
+---
+title: My Slideshow
+parallaxBackgroundImage: /path/to/my/background_image.png
+---
+
+## Slide One
+
+Slide 1 has background_image.png as its background.
+
+## {data-background-image="/path/to/special_image.jpg"}
+
+Slide 2 has a special image for its background, even though the header has no content.
+```
+
 Creating EPUBs with pandoc
 ==========================