update doc

Author: youchenlee

readme.md

@@ -131,9 +131,20 @@ $fa_article = new FileApi('/images/article/'); # initiate another instance
 ### Get image url
 
 ```php
-$fa->get('wfj412.jpg');        // => get image url of 'L' size
-$fa->get('wfj412.jpg', 'M');   // => get image url of 'M' size
-$fa->get('wfj412.jpg', 'full'); // => get image url of full size
+// large size
+$fa->get('wfj412.jpg');
+$fa->get('wfj412.jpg', 'L');
+$fa->get('wfj412.jpg', 'FileApi::SIZE_LARGE');
+
+// medium size
+$fa->get('wfj412.jpg', 'M');
+$fa->get('wfj412.jpg', FileApi::SIZE_MEDIUM);
+
+// full size
+$fa->get('wfj412.jpg', 'full');
+$fa->get('wfj412.jpg', FileApi::SIZE_ORIGINAL);
+
+// comporssed
 $fa->get('wfj412.jpg', 'CP'); // => get image url of compressed one
 ```
 
@@ -234,4 +245,4 @@ you add event listener to finish up after file uploaded, file api will fire `ima
 
     'enable_api_upload' => false, // auto upload api
     'api_prefix' => '/api/v1',    // upload api url prefix
-    'middlewares' => [],          // middlewares that wrap the api upload route
+    'middlewares' => [],          // middlewares that wrap the api upload route