Protocol spec: add references for the NU4 upgrade.

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

protocol/protocol.tex

@@ -8644,6 +8644,9 @@ then \Blossom then \Heartwood\notbeforenufour{ then \Nufour}, and for future upg
 \heartwood{The specifications of the \Heartwood upgrade are described in this document,
 \cite{ZIP-250}, \cite{ZIP-213}, and \cite{ZIP-221}.}
 
+\nufour{The specifications of the \Nufour upgrade are described in this document,
+\cite{ZIP-251}, \cite{ZIP-207}, and \cite{ZIP-214}.}
+
 \vspace{1ex}
 \introlist
 Each \networkUpgrade is introduced as a
@@ -10386,6 +10389,19 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
 \intropart
 \lsection{Change History}{changehistory}
 
+\historyentry{2020.1.3}{}
+
+\begin{itemize}
+    \item No changes to \Sprout, \Sapling, or \Blossom.
+\heartwood{
+    \item Minor wording clarifications.
+}
+\nufour{
+    \item Reference \cite{ZIP-251}, \cite{ZIP-207}, and \cite{ZIP-214} for the \Nufour upgrade.
+}
+\end{itemize}
+
+
 \historyentry{2020.1.2}{2020-03-20}
 
 \begin{itemize}

protocol/zcash.bib

@@ -927,6 +927,15 @@ Last revised February~5, 2018.}
    urldate={2020-03-20}
 }
 
+@misc{ZIP-214,
+   presort={ZIP-0214},
+   author={Daira Hopwood},
+   title={Consensus rules for a Zcash Development Fund},
+   howpublished={Zcash Improvement Proposal 214. Created February~28, 2020.},
+   url={https://zips.z.cash/zip-0214},
+   urldate={2020-03-24}
+}
+
 @misc{ZIP-221,
    presort={ZIP-0221},
    author={Jack Grigg},
@@ -954,6 +963,15 @@ Last revised February~5, 2018.}
    urldate={2020-03-20}
 }
 
+@misc{ZIP-251,
+   presort={ZIP-0251},
+   author={Daira Hopwood},
+   title={Deployment of the \$\{NU4\} Network Upgrade},
+   howpublished={Zcash Improvement Proposal 251. Created February~28, 2020.},
+   url={https://zips.z.cash/zip-0251},
+   urldate={2020-03-24}
+}
+
 @misc{ZIP-302,
    presort={ZIP-0302},
    author={Jay Graber and Jack Grigg},