document the app's activity hierarchy

activities.md

@@ -0,0 +1,16 @@
+# APRSdroid activities and inheritance
+
+* **LoadingListActivity** (Android ListActivity) -- helper to show progress spinner while loading
+  * **ConversationsActivity** -- shows all APRS message conversations
+  * **MainListActivity** -- helper with start/stop action buttons at the bottom
+    * **HubActivity** -- list of stations sorted by distance
+    * **LogActivity** -- list of all packets sent and received
+  * **StationHelper** -- helper for per-station menus
+    * **MessageActivity** -- conversation with a callsign
+    * **StationActivity** -- details of a callsign, list of all SSIDs, list of all packets
+* **MapAct** (MapsForge MapActivity) -- offline map
+* **GoogleMapAct** (Android Activity) -- online map
+* **PrefsAct** (Android PreferenceActivity) -- central preferences 
+* **PrefSymbolAct** (Android Activity) -- chooser for APRS symbol
+* **ProfileImportActivity** (Android Activity) -- JSON profile file import
+* **KeyfileImportActivity** (Android Activity) -- SSL key (.p12) file import