Add details to README on creating themes.

Closes #662.

css/theme/README.md

@@ -2,7 +2,7 @@
 
 Themes are written using Sass to keep things modular and reduce the need for repeated selectors across files. Make sure that you have the reveal.js development environment including the Grunt dependencies installed before proceding: https://github.com/hakimel/reveal.js#full-setup
 
-
+You also need to install Ruby and then Sass (with `gem install sass`).
 
 ## Creating a Theme
 
@@ -21,3 +21,5 @@ This is where you override the default theme. Either by specifying variables (se
 
 4. **Include [/css/theme/template/theme.scss](https://github.com/hakimel/reveal.js/blob/master/css/theme/template/theme.scss)**
 The template theme file which will generate final CSS output based on the currently defined variables.
+
+When you are done, run `grunt themes` to compile the Sass file to CSS and you are ready to use your new theme.