Update README to include API Key instructions.

These instructions are on the google_map_flutter plugin's README, but it is beneficial to have them here, too.

place_tracker/README.md

@@ -4,7 +4,20 @@ A sample place tracking app that uses the [google_maps_flutter](https://github.c
 
 ## Getting Started
 
-For help setting up the plugin, see the plugin's [README](https://github.com/flutter/plugins/tree/master/packages/google_maps_flutter) page.
+To run this sample app, you will need an API key.
+
+Get an API key at <https://cloud.google.com/maps-platform/>.
+
+Specify your API key in the application manifest `android/app/src/main/AndroidManifest.xml`:
+
+```xml
+<manifest ...
+  <application ...
+    <meta-data android:name="com.google.android.geo.API_KEY"
+               android:value="YOUR KEY HERE"/>
+```
+
+For additional help setting up the plugin, see the plugin's [README](https://github.com/flutter/plugins/tree/master/packages/google_maps_flutter) page.
 
 For help getting started with Flutter, view our online
 [documentation](https://flutter.io/).