x/tools/present: add presenter notes documentation

Fixes golang/go#14654

Fixes golang/go#12355

Change-Id: I0c05db624170f7bef5883192b47b618ca343a830
Reviewed-on: https://go-review.googlesource.com/21489
Reviewed-by: Andrew Gerrand <adg@golang.org>

cmd/present/doc.go

@@ -12,6 +12,7 @@ Usage of present:
   -base="": base path for slide template and static resources
   -http="127.0.0.1:3999": HTTP service address (e.g., '127.0.0.1:3999')
   -nacl=false: use Native Client environment playground (prevents non-Go code execution)
+  -notes=false: enable presenter notes (press 'N' from the browser to display them)
   -orighost="": host component of web origin URL (e.g., 'localhost')
   -play=true: enable playground (permit execution of arbitrary user code)
 

present/doc.go

@@ -224,5 +224,35 @@ It is your responsibilty to make sure the included HTML is valid and safe.
 
 	.html file.html
 
+Presenter notes:
+
+Presenter notes may be enabled by appending the "-notes" flag when you run
+your "present" binary.
+
+This will allow you to open a second window by pressing 'N' from your browser
+displaying your slides. The second window is completely synced with your main
+window, except that presenter notes are only visible on the second window.
+
+Lines that begin with ": " are treated as presenter notes.
+
+	* Title of slide
+
+	Some Text
+
+	: Presenter notes (first paragraph)
+	: Presenter notes (subsequent paragraph(s))
+
+Notes may appear anywhere within the slide text. For example:
+
+	* Title of slide
+
+	: Presenter notes (first paragraph)
+
+	Some Text
+
+	: Presenter notes (subsequent paragraph(s))
+
+This has the same result as the example above.
+
 */
 package present // import "golang.org/x/tools/present"