Add a custom icon for the "Index" button.

* res/menu/options_browser.xml: Use a custom icon for the "Index"
  button.
* pr/ic_rtfm.xcf.gz, res/drawable-hdpi/ic_rtfm.png,
  res/drawable-ldpi/ic_rtfm.png, res/drawable-mdpi/ic_rtfm.png,
  res/drawable-xhdpi/ic_rtfm.png: New files.