Add Overwinter ZIPs and update section on Overwinter/Sapling transitions.

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

protocol/protocol.tex

@@ -5631,12 +5631,16 @@ hard fork strategy generally, and then describe the \Sapling
 transition as an instance of that.}
 
 \Zcash launched with a protocol revision that we call \Sprout.
-\Sapling is the next major revision of the protocol, which is still
-in the design phase at the time of writing. This section summarizes
-the planned strategy for upgrading from \Sprout to \Sapling.
+At the time of writing, two upgrades are planned: \NUZero, and
+\Sapling. This section summarizes the planned strategy for upgrading
+from \Sprout to \NUZero and then \Sapling.
 
-\Sapling will be introduced in a ``bilateral hard fork''. In this
-kind of fork,
+The upgrade mechanism is described in \cite{ZIP-200}.
+The content of the \NUZero upgrade is described in \cite{ZIP-201},
+\cite{ZIP-202}, \cite{ZIP-203}, and \cite{ZIP-143}.
+
+\NUZero and \Sapling will each be introduced in a
+``bilateral hard fork''. In this kind of fork,
 
 \begin{enumerate}
   \item there is a \blockHeight at which the fork takes effect;
@@ -5647,8 +5651,8 @@ kind of fork,
         forking \block.
 \end{enumerate}
 
-Full support for \Sapling is indicated by a minimum version of the
-peer-to-peer protocol. Before the planned fork \blockHeight,
+Full support for each upgrade is indicated by a minimum version
+of the peer-to-peer protocol. Before the planned fork \blockHeight,
 nodes that support \Sapling will disconnect from (and will not
 reconnect to) nodes with a protocol version lower than this
 minimum.

protocol/zcash.bib

@@ -582,6 +582,51 @@ Received \mbox{April 13,} 2011.}
    urldate={2018-01-22}
 }
 
+@misc{ZIP-143,
+   presort={ZIP-0143},
+   author={Jack Grigg and Daira Hopwood},
+   title={Transaction Signature Verification for Overwinter},
+   howpublished={Zcash Improvement Proposal 143. Created December 27, 2017.},
+   url={https://github.com/zcash/zips/blob/master/zip-0143.rst},
+   urldate={2018-03-01}
+}
+
+@misc{ZIP-200,
+   presort={ZIP-0200},
+   author={Jack Grigg},
+   title={Network Upgrade Mechanism},
+   howpublished={Zcash Improvement Proposal 200. Created January 8, 2018.},
+   url={https://github.com/zcash/zips/blob/master/zip-0200.rst},
+   urldate={2018-03-01}
+}
+
+@misc{ZIP-201,
+   presort={ZIP-0201},
+   author={Simon Liu},
+   title={Network Peer Management for Overwinter},
+   howpublished={Zcash Improvement Proposal 201. Created January 15, 2018.},
+   url={https://github.com/zcash/zips/blob/master/zip-0201.rst},
+   urldate={2018-03-01}
+}
+
+@misc{ZIP-202,
+   presort={ZIP-0202},
+   author={Simon Liu},
+   title={Version 3 Transaction Format for Overwinter},
+   howpublished={Zcash Improvement Proposal 202. Created January 10, 2018.},
+   url={https://github.com/zcash/zips/blob/master/zip-0202.rst},
+   urldate={2018-03-01}
+}
+
+@misc{ZIP-203,
+   presort={ZIP-0203},
+   author={Jay Graber},
+   title={Transaction Expiry},
+   howpublished={Zcash Improvement Proposal 203. Created January 9, 2018.},
+   url={https://github.com/zcash/zips/blob/master/zip-0203.rst},
+   urldate={2018-03-01}
+}
+
 @misc{DigiByte-PoW,
    presort={DigiByte-PoW},
    author={DigiByte Core Developers},