include preface in TOC

drbruced12 · drbruced12 · commit f214db2c191d · 2025-07-18T17:02:51.000+10:00
diff --git a/index.rst b/index.rst
@@ -18,6 +18,7 @@ Peterson and Davie
    :caption: Table of Contents
    :glob:
 
+   preface.rst
    intro.rst
    principles.rst
    crypto.rst
