add description of app to manifest for p2p users

The local repo generation code will take the description from the manifest
and include it in the repo meta data.  So FDroid itself should also include
this description.  Indeed every app should... perhaps this should also be
an `fdroid lint` item.

AndroidManifest.xml

@@ -54,6 +54,7 @@
         android:name="FDroidApp"
         android:icon="@drawable/ic_launcher"
         android:label="@string/app_name"
+        android:description="@string/app_description"
         android:allowBackup="true"
         android:theme="@style/AppThemeDark"
         android:supportsRtl="true" >

res/values/strings.xml

@@ -281,4 +281,5 @@
     <string name="system_permission_denied_title">System permissions denied</string>
     <string name="system_permission_denied_body">This option is only available when F-Droid is installed as a system-app.</string>
 
+    <string name="app_description">F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.</string>
 </resources>