clarify README

* default settings can be done before the document
  ready event
* tagcloud() needs to be called on a selector
  matching each item of the tag cloud

README.md

@@ -12,11 +12,13 @@ Usage
 
 and then:
 
-    $(document).ready(function(){
-      $.fn.tagcloud.defaults = {
-        size: {start: 14, end: 18, unit: "pt"},
-        color: {start: '#cde', end: '#f52'}
-      };
+    $.fn.tagcloud.defaults = {
+      size: {start: 14, end: 18, unit: 'pt'},
+      color: {start: '#cde', end: '#f52'}
+    };
+
+    $(function () {
+      $('#whatever a').tagcloud();
     });
 
 Demo