[image_picker] Add documentation for Android external storage permissions (flutter#2765)

* update README with external storage configuration info

* update pubspec.yaml version

* format change

Author: cg021

packages/image_picker/image_picker/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.6.6+3
+
+* Update README
+
 ## 0.6.6+2
 
 * Update lower bound of dart dependency to 2.1.0.

packages/image_picker/image_picker/README.md

@@ -19,8 +19,13 @@ Add the following keys to your _Info.plist_ file, located in `<project root>/ios
 
 ### Android
 
+#### API 29+
 No configuration required - the plugin should work out of the box.
 
+#### API < 29
+
+Add `android:requestLegacyExternalStorage="true"` as an attribute to the `<application>` tag in AndroidManifest.xml. The [attribute](https://developer.android.com/training/data-storage/compatibility) is `false` by default on apps targeting Android Q. 
+
 ### Example
 
 ``` dart

packages/image_picker/image_picker/pubspec.yaml

@@ -2,7 +2,7 @@ name: image_picker
 description: Flutter plugin for selecting images from the Android and iOS image
   library, and taking new pictures with the camera.
 homepage: https://github.com/flutter/plugins/tree/master/packages/image_picker/image_picker
-version: 0.6.6+2
+version: 0.6.6+3
 
 flutter:
   plugin: