transcript view: list button for touch devices

The 'r' keyboard shortcut is useless on mobile browsers, so the
transcript view can now also be toggled with a button next to the
speaker notes button (shown on coarse-pointer devices, like the
fullscreen and notes buttons, and highlighted while the transcript is
open).

Change-Id: I10909124850a5fba469a670115e87ef4f6c4376b
diff --git a/README.Rmd b/README.Rmd
index 8c64a12..d5611cd 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -109,7 +109,7 @@
 
 -   `'s'` enable presentation mode with speaker notes when the Notes plugin is activated
 
-On mobile browsers (touch devices), a speech-bubble button next to the fullscreen button overlays the current slide with its speaker notes -- useful for practicing a talk on a phone or tablet, where the separate speaker view window of the `'s'` shortcut is awkward. The overlay shows the slide title, its planned time (if annotated with a timing comment), and the notes, and follows along as you change slides. It appears only if the deck contains notes at all.
+On mobile browsers (touch devices), a speech-bubble button next to the fullscreen button overlays the current slide with its speaker notes -- useful for practicing a talk on a phone or tablet, where the separate speaker view window of the `'s'` shortcut is awkward. The overlay shows the slide title, its planned time (if annotated with a timing comment), and the notes, and follows along as you change slides. It appears only if the deck contains notes at all. Next to it, a list button opens the transcript view (same as pressing `r`), so the whole talk including notes can be read (aloud) on mobile devices, too.
 
 -   `'r'` toggle the transcript view: a single scrollable page containing every slide's content and speaker notes, so text-to-speech ("read aloud") browser extensions can read the whole presentation -- they cannot walk a reveal.js deck themselves, since hidden slides are not part of the visible page text