Update README.

Author: geoffp

README.md

@@ -1,3 +1,5 @@
-## The Handlebars engine for Pattern Lab / Node
+## The Underscore engine for Pattern Lab / Node
 
-To install the Handlebars engine in your edition, `npm install patternengine-node-handlebars` should do the trick.
+To install the Underscore engine in your edition, `npm install patternengine-node-underscore` should do the trick.
+
+Level of support is basic; you can author patterns in Underscore, but support for calling pattern partials is accomplished through an Underscore mixin, and is considered experimental.