Add section on depends-on snippets

emacs/.emacs.d/org/intro.org

@@ -42,3 +42,20 @@ feed]]), which is where I keep my configuration.
   prefix all the functions I write and variables I declare with ~oni:~.
   You don't have to copy it if you copy anything from this file, as
   long as you do it consistently.
+
+* The ~depends-on~ snippets
+
+  At certain points in this document you will find bits of code that
+  look like:
+
+  #+BEGIN_SRC emacs-lisp :tangle no
+    (depends-on "somepackage")
+  #+END_SRC
+
+  These calls are from the Cask DSL. They specify that my
+  configuration depends on some package found in one of the ELPA
+  repositories I have setup.
+
+  These snippets don't belong in your =init.el=, but instead in a
+  separate =Cask= file, and only if you use Cask, not if you just use
+  the standard ~package.el~.