add info on split APK installers

static/usage.html

@@ -704,7 +704,9 @@ <h2><a href="#sandboxed-play-services">Sandboxed Play services</a></h2>
                 <p>Install com.google.android.gsf, then com.google.android.gms and finally use a
                 split APK installer to install all 5 of the APKs for com.android.vending together.
                 Make sure to install all 3 in the correct order and don't skip installing the Play
-                Store since it provides services used by apps depending on Play.</p>
+                Store since it provides services used by apps depending on Play. You can obtain a
+                split APK installer app from either Aurora Store, F-Droid or the developers of the
+                app via their GitHub releases, etc.</p>
 
                 <p>In the future, we'll have a client app for our repository so you'll be able to
                 install and update the official Play services apps through that app and you won't