Merge pull request #99 from CatalystCode/thcao/android-docs-update

Minor Android docs update.

Author: phongcao

docs/android-installation.md

@@ -188,7 +188,11 @@ const connectionString = '...';       // The Notification Hub connection string
 const hubName = '...';                // The Notification Hub name
 const senderID = '...';               // The Sender ID from the Cloud Messaging tab of the Firebase console
 const tags = [ ... ];                 // The set of tags to subscribe to
-const channelImportance = 3;    	    // The channel's importance (IMPORTANCE_DEFAULT = 3)
+const channelImportance = 3;          // The channel's importance (NotificationManager.IMPORTANCE_DEFAULT = 3)
+                                      // Notes:
+                                      //   1. Setting this value to 4 enables heads-up notification on Android 8
+                                      //   2. On some devices such as Samsung Galaxy, changing this value requires
+                                      //      uninstalling/re-installing the app to take effect.
 const channelShowBadge = true;
 const channelEnableLights = true;
 const channelEnableVibration = true;