Add references for the halo2 book.

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

protocol/protocol.tex

@@ -10681,7 +10681,7 @@ Define $\sqrtratioG(\num, \xdiv) \typecolon \GF{\ParamG{q}} \times \GFstar{\Para
   \item An arbitrary square root may be chosen in either case of the definition. The result is never $\bot$.
   \item The choice of the nonsquare $\ParamG{\lambda}$ is also arbitrary and will not affect the output
         of $\maptocurvesimpleswuIsoG$ defined below.
-  \item The computation of $\sqrtratioG$ can be optimized as described in \todo{}.
+  \item The computation of $\sqrtratioG$ can be optimized as described in \cite[section 3.2.1 Fields]{Zcash-halo2}.
 \end{nnotes}
 
 Define $\ParamIsoG{Z} := -13 \pmod{\ParamG{q}}$. (This value is suitable for both \IsoPallas and \IsoVesta.)
@@ -10946,7 +10946,7 @@ verifier \MUST check, for the encoding of each element, that:
 \lsubsubsubsection{\HaloTwoText}{halo2}
 
 For \Orchard \actionDescriptions in version 5 \transactions, \Zcash uses \zkSNARKs with the
-\defining{\HaloTwo} \provingSystem described in \todo{}.
+\defining{\HaloTwo} \provingSystem described in \cite{Zcash-halo2}.
 
 \introlist
 \lsubsubsubsubsection{Encoding of \HaloTwoText{} Proofs}{halo2encoding}
@@ -13987,6 +13987,7 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
                   between the protocol specification and \cite{ZIP-225}.
             \item Make the naming of $\enableSpends$ and $\enableOutputs$ consistent.
           \end{itemize}
+    \item Add references to \cite{Zcash-halo2}.
     \item Correct the description of $\lengthField$ in \crossref{unifiedpaymentaddrencoding}.
     \item Correct the type signature of $\DiversifyHash{Orchard}$ in \crossref{abstracthashes}.
     \item Various rationale updates for \NUFive.

protocol/zcash.bib

@@ -1840,3 +1840,11 @@ Proceedings of the 19th Annual International Cryptology Conference
    url={https://zcash.github.io/orchard/},
    urldate={2021-03-02}
 }
+
+@misc{Zcash-halo2,
+   presort={Zcash-halo2},
+   author={Daira Hopwood and Sean Bowe and Jack Grigg and Kris Nuttycombe and Ying Tong Lai and Steven Smith},
+   title={The halo2 Book},
+   url={https://zcash.github.io/halo2/},
+   urldate={2021-03-23}
+}