Add help dialog to feed screen

Help to a possible confusion or simply awareness that NewPipe has both
ways to load the feed.

app/src/main/res/values/attrs.xml

@@ -80,6 +80,7 @@
     <attr name="ic_telescope" format="reference"/>
     <attr name="ic_megaphone" format="reference"/>
     <attr name="ic_sort" format="reference"/>
+    <attr name="ic_help" format="reference"/>
 
     <attr name="progress_horizontal_drawable" format="reference"/>
     <!-- Can't refer to colors directly in drawable's xml-->