Added mention of answer repository to preface and acknowledgements

src/plfa/backmatter/acknowledgements.md

@@ -38,6 +38,16 @@ $endfor$
 </div>
 $endif$
 
+For contributions to the answers repository:
+<div class="list-of-contributors">
+<ul>
+<li>William Cook</li>
+<li>David Banas</li>
+</ul>
+</div>
+There is a private repository of answers to selected questions on
+github. Please contact Philip Wadler if you would like to access it.
+
 For support:
 
   - EPSRC Programme Grant EP/K034413/1

src/plfa/frontmatter/preface.md

@@ -114,3 +114,5 @@ You may need to import library functions required for the solution.
 
 Please do not post answers to the exercises in a public place.
 
+There is a private repository of answers to selected questions on
+github. Please contact Philip Wadler if you would like to access it.