Add prompt to role spinner. Would really like to add hints to each

element but don't know how to do that yet.

xwords4/android/XWords4/res/layout/game_config.xml

@@ -34,6 +34,7 @@
              android:layout_height="wrap_content"
              android:drawSelectorOnTop="true"
              android:entries="@array/role_names"
+             android:prompt="@string/role_prompt"
              />
 
     <Spinner android:id="@+id/connect_spinner"

xwords4/android/XWords4/res/values/strings.xml

@@ -195,6 +195,8 @@
     <string name="role_standalone">Standalone</string>
     <string name="role_host">Host</string>
     <string name="role_guest">Guest</string>
+    <string name="role_prompt">Standalone if all players are on this
+    device; otherwise Host a game or be a Guest.</string>
 
     <string name="phonies_ignore">Ignore phonies</string>
     <string name="phonies_warn">Warn if phonies</string>