Add docs for Android single mode.

Author: cnjsstong

README.md

@@ -42,6 +42,7 @@ Currently, [RxGalleryFinal](https://github.com/FinalTeam/RxGalleryFinal) is stil
 
 ```javascript
 import MultipleImagePicker from 'react-native-multiple-image-picker';
+
 const options = {
     maxImagesCount: 9,      // Max number of images user can select
     selectedPaths: [
@@ -51,5 +52,16 @@ const options = {
         '/Users/tshen/Library/Developer/CoreSimulator/Devices/8C416B45-F555-4A63-A1B0-09E61109F0A0/data/Containers/Data/Application/A1790255-CDE8-486C-A6BA-1693BA2AA87B/Documents/BB6ADD56-09E7-402C-BF0E-AD79400D3889-7539-000007B93A6B5733/3.jpg'
     ]                       // Currently selected paths, must be from result of previous calls
 };
-MultipleImagePicker.launchImageGallery(options);
+MultipleImagePicker.launchImageGallery(options).then((newSelectedPaths) => {
+    // newSelectedPaths will be an Array of String, like [ '/path/1', '/path/2' ], and may be used for `selectedPaths` on the next invocation
+});
+
+// single mode - Android only
+const singleOptions = {
+    single: true
+};
+MultipleImagePicker.launchImageGallery(singleOptions).then((path) => {
+    // path will be a String, like 'file:///storage/sdcard/Download/IMG_0617.JPG'
+});
+
 ```