readme update

README.md

@@ -5,10 +5,22 @@ A cache object that deletes the least-recently-used items.
 Usage:
 
     var LRU = require("lru-cache")
-      , cache = LRU(10) // max 10 items. default = Infinity
+      , cache = LRU(10, // max length. default = Infinity
+                    // calculate how "big" each item is
+                    //
+                    // defaults to function(){return 1}, ie, just limit
+                    // the item count, without any knowledge as to their
+                    // relative size.
+                    function (item) { return item.length })
+
     cache.set("key", "value")
     cache.get("key") // "value"
 
     cache.reset()    // empty the cache
 
+If you put more stuff in it, then items will fall out.
+
+If you try to put an oversized thing in it, then it'll fall out right
+away.
+
 RTFS for more info.