docs: state default plugin set and reframe stock themes as rarely used alternatives

Change-Id: Id5a37a34b4351fe17fa5c4d32572db6da2d44f04
diff --git a/README.Rmd b/README.Rmd
index d5611cd..aa034de 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -238,7 +238,7 @@
 
 There are several options that control the appearance of revealjs presentations:
 
--   `theme` specifies the theme to use for the presentation (available themes are `r knitr::combine_words(setdiff(revealjs.ids:::revealjs_themes(), "default"), before = '"', and = " or ")`
+-   `theme` specifies the visual theme for the presentation. It defaults to the dedicated IDS corporate design theme (`"ids"`), which is the raison d'être of this package; the stock reveal.js themes are listed here only for completeness, as alternatives that will rarely be used in the IDS context: `r knitr::combine_words(setdiff(revealjs.ids:::revealjs_themes(), c("default", "ids")), before = '"', and = " or ")` (`"dark"` is an alias for `"black"`).
 
 -   `highlight` specifies the syntax highlighting style. Supported styles include `r knitr::combine_words(rmarkdown:::highlighters(), before = '"', and = " or ")`. Pass null to prevent syntax highlighting.
 
@@ -249,12 +249,11 @@
 ``` yaml
 output:
   revealjs.ids::revealjs_presentation:
-    theme: sky
     highlight: pygments
     center: true
 ```
 
-[Revealjs documentation about themes](https://revealjs.com/themes/)
+Documentation of the rarely used stock themes: [reveal.js themes](https://revealjs.com/themes/)
 
 ## Slide Transitions
 
@@ -477,7 +476,9 @@
 | [chalkboard](https://github.com/rajgoel/reveal.js-plugins/tree/master/chalkboard)  | Include handwritten notes within a presentation. Press `c` to write on slides, Press `b` to open a whiteboard or chalkboard to write. |
 | [menu](https://github.com/denehyg/reveal.js-menu)                                  | Include a navigation menu within a presentation. Press `m` to open the menu.                                                          |
 
-Note that the use of plugins requires that the `self_contained` option be set to false. For example, this presentation includes both the "notes" and "search" plugins:
+The plugins `notes`, `search`, `menu`, and `zoom` are activated by default; only `chalkboard` is opt-in. Setting `reveal_plugins` yourself replaces the default set, so to add `chalkboard` while keeping the defaults, list them all explicitly. Set `reveal_plugins: null` to render without any plugins.
+
+Note that the use of plugins requires that the `self_contained` option be set to false. For example, this presentation uses only the "notes" and "search" plugins (dropping "menu" and "zoom" from the default set):
 
 ``` yaml
 ---
@@ -584,7 +585,6 @@
 
 ``` yaml
 revealjs.ids::revealjs_presentation:
-  theme: sky
   transition: fade
   highlight: pygments
 ```