doc/tutorial/basics: add per-page chapter headings

Change-Id: I9e0a6b5d1a1c820081b3f87c3871068ea042f889
diff --git a/doc/tutorial/basics/Readme.md b/doc/tutorial/basics/Readme.md
index 0a75cc7..fa0deb7 100644
--- a/doc/tutorial/basics/Readme.md
+++ b/doc/tutorial/basics/Readme.md
@@ -24,7 +24,7 @@
   - [Number Literals](numberlit.md)
   - [String Literals](stringlit.md)
   - [Bytes](bytes.md)
-- Scoping
+- References and Visibility
   - [References and Scopes](scopes.md)
   - [Accessing Fields](selectors.md)
   - [Aliases](aliases.md)
@@ -34,8 +34,9 @@
   - [Duplicate Fields](duplicates.md)
   - [Bottom / Error](bottom.md)
   - [Basic Types](types.md)
-  - [Unification](unification.md)
+  - [Order is Irrelevant](unification.md)
   - [Disjunctions](disjunctions.md)
+  - [Default Values](defaults.md)
   - [Disjunctions of Structs](disjstruct.md)
   - [Numbers](numbers.md)
   - [Ranges](ranges.md)