add note about the basewiki being self contained

doc/basewiki.mdwn

@@ -18,3 +18,9 @@ It currently includes these pages:
 
 As well as a few other files, like [[favicon.ico]], [[local.css]],
 [[style.css]], and some icons.
+
+Note that an important property of the basewiki is that it should be
+self-contained. That means that the pages listed above cannot link
+to pages outside the basewiki. Ikiwiki's test suite checks that the
+basewiki is self-contained, and from time to time links have to be
+removed (or replaced with `iki` [[shortcuts]]) to keep this invariant.