Mark partials as optional

Author: Tomek Wiszniewski

README.md

@@ -119,7 +119,7 @@ Following is an [rtype](https://git.io/rtype) signature of the most commonly use
 Mustache.render(
   template  : String,
   view      : Object,
-  partials  : Object,
+  partials? : Object,
 ) => String
 
 Mustache.parse(