fix bug

Author: wxxiong6

README.md

@@ -35,8 +35,8 @@ ArrayList implements ArrayAccess {
 ## More
 ```php
 $ret = new ArrayList();
-$ret->add(1);
-$ret->get(0);
+$ret[] = 1; // 也可以使用 $ret->add(1); 
+$ret[0];    // 也可以使用 $ret->get(0);
 $ret->count();
 $ret->getSize();
 $ret->toArray();
@@ -72,3 +72,28 @@ for ($i = 0; $i< $ret->count(); $i++) {
     }
 }
 ```
+## 与数组写入性能对比
+
+Testing size: 1,000
+ArrayList():     time:0.00059604644775391 memory:16.1875
+Array():         time:0.00033903121948242 memory:19.9375
+
+Testing size: 2,000
+ArrayList():     time:0.00047087669372559 memory:32.1875
+Array():         time:0.00060915946960449 memory:35.9375
+
+Testing size: 4,000
+ArrayList():     time:0.00081586837768555 memory:64.1875
+Array():         time:0.000762939453125 memory:67.9375
+
+Testing size: 8,000
+ArrayList():     time:0.00094485282897949 memory:128.1875
+Array():         time:0.0012369155883789 memory:131.9375
+
+Testing size: 2,048,000
+ArrayList():     time:0.19431805610657 memory:32000.28125
+Array():         time:0.32430100440979 memory:33539.9375
+
+Testing size: 4,096,000
+ArrayList():     time:0.44307684898376 memory:64000.28125
+Array():         time:0.67080807685852 memory:67075.9375