added mini roadmap during implementation of background play

Background-Playback-Mini-Roadmap.md

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+1. Media playback on `IntentService`'s worker thread
+    - wifilock/wakelock
+    - requires `INTERNET` permission,
+    - wakelock permission too
+2. Material Notification-based playback controls
+    - running in foreground - calls `startForeground()`
+    - uses category `CATEGORY_TRANSPORT` (I think?)
+    - classed as a `LOW` priority notification in material notification design
+3. Listen for changes in audio focus
+
+A lot of these suggestions are taken from <http://developer.android.com/design/patterns/notifications.html>