(#7) Add SAF API to README.md docs

Author: LaksCastro

README.md

@@ -9,6 +9,7 @@ Plugin to fetch Android shared storage/folders info
 - _**Android Only**_
 - _**Alpha version**_
 - _**Supports Android 4.1+ (API Level 16+)**_
+- _**The `targetSdk` should be set to `31`**_
 
 ### Features
 
@@ -19,8 +20,8 @@ This plugin allow us to get path of top-level shared folder (Downloads, DCIM, Vi
 ```dart
 /// Get Android [downloads] top-level shared folder
 /// You can also create a reference to a custom directory as: `EnvironmentDirectory.custom('Custom Folder')`
-final sharedDirectory = 
-    await getExternalStoragePublicDirectory(EnvironmentDirectory.downloads); 
+final sharedDirectory =
+    await getExternalStoragePublicDirectory(EnvironmentDirectory.downloads);
 
 print(sharedDirectory.path); /// `/storage/emulated/0/Download`
 ```
@@ -29,26 +30,74 @@ print(sharedDirectory.path); /// `/storage/emulated/0/Download`
 
 ```dart
 /// Get Android [downloads] shared folder for Android 9+
-final sharedDirectory = 
+final sharedDirectory =
     await getMediaStoreContentDirectory(MediaStoreCollection.downloads);
 
 print(sharedDirectory.path); /// `/external/downloads`
 ```
 
-- Get root Android path, note that is a read-only folder
+- Start `OPEN_DOCUMENT_TREE` activity to prompt user to select an folder to enable write and read access to be used by the `Storage Access Framework` API
 
 ```dart
-/// Get Android root folder
-final sharedDirectory = await getRootDirectory();
+/// Get permissions to manage an Android directory
+final selectedUriDir = await openDocumentTree();
 
-print(sharedDirectory.path); /// `/system`
+print(selectedUriDir);
+```
+
+- Create a new file using the `SAF` API
+
+```dart
+/// Create a new file using the `SAF` API
+final newDocumentFile = await createDocumentFile(
+  mimeType: '	text/plain',
+  content: 'My Plain Text Comment Created by shared_storage plugin',
+  displayName: 'CreatedBySharedStorageFlutterPlugin',
+  directory: anySelectedUriByTheOpenDocumentTreeAPI,
+);
+
+print(newDocumentFile);
+```
+
+- Get all persisted [URI]s by the `openDocumentTree` API, from `SAF` API
+
+```dart
+/// You have [write] and [read] access to all persisted [URI]s
+final listOfPersistedUris = await persistedUriPermissions();
+
+print(newDocumentFile);
+```
+
+- Revoke a current persisted [URI], from `SAF` API
+
+```dart
+/// Can be any [URI] returned by the `persistedUriPermissions`
+final uri = ...;
+
+/// After calling this, you no longer has access to the [uri]
+await releasePersistableUriPermission(uri);
+
+print(newDocumentFile);
+```
+
+- Convenient method to know if a given [uri] is a persisted `uri` ("persisted uri" means that you have `write` and `read` access to the `uri` even if devices reboot)
+
+```dart
+/// Can be any [URI], but the method will only return [true] if the [uri]
+/// is also present in the list returned by `persistedUriPermissions`
+final uri = ...;
+
+/// Verify if you have [write] and [read] access to a given [uri]
+final isPersisted = await isPersistedUri(uri);
+
+print(newDocumentFile);
 ```
 
 ### Android API's
 
 Most Flutter plugins uses Android API's under the hood. So this plugin do the same, and to retrieve Android shared folder paths the following API's are being used:
 
-[`🔗 android.os.Environment`](https://developer.android.com/reference/android/os/Environment#summary) [`🔗 android.provider.MediaStore`](https://developer.android.com/reference/android/provider/MediaStore#summary)
+[`🔗android.os.Environment`](https://developer.android.com/reference/android/os/Environment#summary) [`🔗android.provider.MediaStore`](https://developer.android.com/reference/android/provider/MediaStore#summary) [`🔗android.provider.DocumentsProvider`](https://developer.android.com/guide/topics/providers/document-provider)
 
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