Introduction: note security fixes, and reference the "Differences from Zerocash" section

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -44,8 +44,8 @@
 \DefineBibliographyStrings{english}{
   page  = {page},
   pages = {pages},
-  backrefpage = {$\uparrow$},
-  backrefpages = {$\uparrow$}
+  backrefpage = {$\uparrow$ p\!},
+  backrefpages = {$\uparrow$ p\!}
 }
 
 \setlength{\oddsidemargin}{-0.25in}
@@ -479,13 +479,14 @@
 \nsection{Introduction}
 
 \Zcash is an implementation of the \term{Decentralized Anonymous Payment}
-scheme \Zerocash \cite{BCG+2014} with some adjustments to terminology,
-functionality and performance. It bridges the existing \emph{transparent}
-payment scheme used by \Bitcoin with a \emph{confidential} payment scheme
-protected by zero-knowledge succinct non-interactive arguments of knowledge
-(\zkSNARKs).
+scheme \Zerocash \cite{BCG+2014}, with some security fixes and adjustments
+to terminology, functionality and performance. It bridges the existing
+\emph{transparent} payment scheme used by \Bitcoin with a \emph{confidential}
+payment scheme protected by zero-knowledge succinct non-interactive arguments
+of knowledge (\zkSNARKs).
 
-Changes from the original \Zerocash are highlighted in \changed{\changedcolor}.
+Changes from the original \Zerocash are explained in \crossref{differences},
+and highlighted in \changed{\changedcolor} throughout the document.
 
 This specification is structured as follows:
 
@@ -2355,7 +2356,7 @@ Unlike \Bitcoin, the difficulty adjustment occurs after every block.
 \todo{Describe the algorithm.}
 
 
-\nsection{Differences from the Zerocash paper}
+\nsection{Differences from the Zerocash paper} \label{differences}
 
 \nsubsection{Transaction Structure} \label{trstructure}