Add documentation for pre-installed apps

Author: wellle

doc/preinstalled.md

@@ -0,0 +1,23 @@
+## Using AdjustIo for pre-installed apps
+
+If you want to use the AdjustIo SDK to recognize users that found your app pre-installed on their device, follow these steps.
+
+1. Integrate AdjustIo as described in our [README].
+2. Create a new tracker in your [dashboard].
+3. Open your `AndroidManifest.xml` and add the following line to your AdjustIo settings:
+
+    ```xml
+    <meta-data android:name="AdjustIoDefaultTracker" android:value="{Tracker}" />
+    ```
+
+    Replace `{Tracker}` with the tracker you created in step 2.
+
+4. Build and run your app. You should see a line like the following in LogCat:
+
+    ```
+    Default tracker: 'abc123'
+    ```
+
+
+[README]: ../README.md
+[dashboard]: http://adjust.io