updated readme draft.

Author: m4gr3d

README.md

@@ -4,7 +4,8 @@
 
 # DroidPlanner
 
-Droidplanner is a Ground Control Station (GCS) Android app for UAVs running Ardupilot software.
+Droidplanner is a Ground Control Station (GCS) Android app built atop [3DR Services](https://github.com/DroidPlanner/3DRServices) for UAVs
+running Ardupilot software.
 
 [![Google Play Store](https://developer.android.com/images/brand/en_app_rgb_wo_45.png)](https://play.google.com/store/apps/details?id=org.droidplanner)
 
@@ -26,6 +27,11 @@ The next version of Droidplanner is under development, and we would love your he
  (It may take 30+ minutes to show up after becoming a beta tester).
  3. Send feedback: Post your feedback in the [Droidplanner Google+ community](https://plus.google.com/communities/109498440846585781402) or the [Droidplanner mailing list](https://groups.google.com/forum/#!forum/droidplanner). We want to hear what you think!
 
+#### Requirement
+In order to run **DroidPlanner v3** on your device, the latest version of **3DR Services** must
+be installed.
+To do so, follow the instructions on the [3DR Services github page](https://github.com/DroidPlanner/3DRServices).
+
 ### Contributing
 
 Droidplanner v3 is in active development. If you would like to contribute to the project,