overview TOC: exclude References and Appendix chapters

Chapters titled References, Literatur, Literaturverzeichnis, Appendix,
or Anhang (and their sub-slides) are now left out of the overview table
of contents.

Change-Id: I2d4c69ef8cb925113b43c3615416091f463bb8ef
diff --git a/R/revealjs_presentation.R b/R/revealjs_presentation.R
index 75d6c23..dab04c7 100644
--- a/R/revealjs_presentation.R
+++ b/R/revealjs_presentation.R
@@ -82,6 +82,18 @@
 #' The function [slide_timing()] computes these totals directly from an
 #' `.Rmd` source file without rendering it.
 #' 
+#' ## Overview slides
+#' 
+#' Like in the original Google Docs slide workflow, a slide titled
+#' "Overview" or "Überblick" is automatically turned into a table of
+#' contents: its body content is replaced at presentation time with a
+#' clickable list of all `#` chapter headings. Adding
+#' `{data-overview-subheadings=true}` to the heading also includes the
+#' `##` sub-headings below their chapters, laid out in two columns.
+#' References and appendix chapters (References/Literatur/
+#' Literaturverzeichnis/Appendix/Anhang) are left out, together with
+#' their sub-slides.
+#' 
 #' ### Search
 #' 
 #' When opt-in, it is possible to show a search box when pressing `CTRL + SHIFT +