Add storage permission section for Android on readme (#150)

Author: rashidul-hasan

README.md

@@ -47,6 +47,14 @@ new MainReactPackage(),
 new RNHTMLtoPDFPackage()
 ```
 
+- Add the following `WRITE_EXTERNAL_STORAGE` permission to `AndroidManifest.xml`
+
+```xml
+<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
+```
+Also starting from Android M, users need to be prompted for permission dynamically. Follow [this](https://facebook.github.io/react-native/docs/permissionsandroid) link for more details on how to do that.
+
+
 ## Usage
 ```javascript