update README

README.md

@@ -32,21 +32,17 @@ alimask('王小为(小为) 888888', { color: 'green' });
 
 # 2. API
 
-The unique API is: **alimask(ext, options)**.
-
-width: 250,
-      height: 80,
-      color: '#ebebeb',
-      fillColor: 'white',
-      font: '10px Arial'
-
- - **width** (Number): default is `250`.
- - **height** (Number): default is `80`
- - **color** (String): the text color, default is `#ebebeb`.
- - **fillColor** (String): the background color, default is `white`.
- - **font** (String): the text font style, default is `10px Arial`.
-
-The api return **the base64 string of water mask image**.
+The unique API is: **alimask(text, options)**.
+
+ - **text** (String): required, the text in the watermark image. 
+ - **options** (Object): optional, the options of watermark, with keys below:
+   - **width** (Number): default is `250`.
+   - **height** (Number): default is `80`
+   - **color** (String): the text color, default is `#ebebeb`.
+   - **fillColor** (String): the background color, default is `white`.
+   - **font** (String): the text font style, default is `10px Arial`.
+
+The api return **the base64 string of watermark image** which can be used in css background / img tag.
 
 
 # 3. Build & Test