update doc

doc/SHARE.md

@@ -57,6 +57,7 @@ However, developers must care about the following points:
     ));
 ```
 ## Share Image
+
 ```dart
  fluwx.share(WeChatShareImageModel(
         image: _imagePath,
@@ -67,6 +68,19 @@ However, developers must care about the following points:
 ```
 >  NOTE：`Fluwx` will create thumbnail from `image` if thumbnail isn't provided.
 
+Well,let's talk about sharing big images on Android,if the image you want to share is smaller than 512k (the truth is the data passed through
+`Intent`  must be smaller than 512k), everything works well.However, if it's bigger than
+512k, take care the following permission:
+```xml
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
+```
+`Fluwx` can't share big images without  the permission above because these images will stored in:
+
+```kotlin
+ context.getExternalCacheDir()
+```
+Why? Because the limit of `Intent`. If someone knows a better solution,just PR or tell me.
+
 ## Share Music
 ```dart
   var model = WeChatShareMusicModel(

doc/SHARE_CN.md

@@ -57,6 +57,20 @@
 
 > 注意：如果不指定 `thumbnail`，那么`Fluwx`将尝试从`image`中获取缩略图。
 
+
+好吧，让我们谈谈在Android上分享大图，如果说你要分享的图片小于512k（实际上是因为`Intent`传值是不能超过512k的），一切都可以正常工作。
+但是如果你要分享的图片大于512k,那么请保证你的app有以下的权限：
+
+```xml
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
+```
+如果`Fluwx`没有以上的权限，那么是无法完成分享的，因为要分享的图片将会被存储在以下位置:
+
+```kotlin
+ context.getExternalCacheDir()
+```
+为什么? 因为`Intent`有限制. 如果有人知道更好的解决方案,请直接PR或者告诉我。
+
 ## 分享音乐
 
 ```dart