add documentation

docs/tutorials/drm-config.md

@@ -209,6 +209,62 @@ you should provide an empty string as robustness
 
 Values for other key systems are not known to us at this time.
 
+#### Re-use persistent license DRM for online playback
+
+If your DRM provider configuration allows you to deliver persistent license,
+you could re-use the created MediaKeys session for the next online playback.
+
+Configure Shaka to start DRM sessions with the `persistent-license` type
+instead of the `temporary` one:
+
+```js
+player.configure({
+  drm: {
+    advanced: {
+      'com.widevine.alpha': {
+        'sessionType': 'persistent-license'
+      }
+    }
+  }
+});
+```
+
+**Using `persistent-license` might now work on every devices, use this feature
+carefully.**
+
+When the playback starts, you can retrieve the sessions metadata:
+
+```js
+const activeDrmSessions = this.player.getActiveSessionsMetadata();
+const persistentDrmSessions = activeDrmSessions.filter(
+  ({ sessionType }) => sessionType === 'persistent-license');
+
+// Add your own storage mechanism here, give it an unique known identifier for
+// the playing video
+```
+
+When starting the same video again, retrieve the metadata from the storage, and
+set it back to Shaka's configuration.
+
+Shaka will load the given DRM persistent sessions and will only request a
+license if some keys are missing for the content.
+
+```js
+player.configure({
+  drm: {
+    persistentSessionOnlinePlayback: true,
+    persistentSessionsMetadata: [{
+      sessionId: 'deadbeefdeadbeefdeadbeefdeadbeef',
+      initData: new InitData(0),
+      initDataType: 'cenc'
+    }]
+  }
+});
+```
+
+NB: Shaka doesn't provide a out-of-the-box storage mechanism for the sessions
+metadata.
+
 #### Continue the Tutorials
 
 Next, check out {@tutorial license-server-auth}.