Describe the recommended way to encode a Sapling or unified payment address as a QR code.

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

protocol/protocol.tex

@@ -11080,6 +11080,12 @@ compatibility with software that only accepts previous address formats is requir
 A \unifiedPaymentAddress is encoded with \Bechm \cite{BIP-350} rather than \Bech.
 } %nufive
 
+\vspace{1ex}
+\xPaymentAddresses \MAY be encoded as QR codes; in this case, the \RECOMMENDED format
+for a \Sapling\nufive{ or unified} \paymentAddress is the \Bech\nufive{ or \Bechm}
+form converted to uppercase, using the Alphanumeric mode \cite[sections 7.3.4 and 7.4.4]{ISO2015}.
+} %sapling
+
 
 \introsection
 \lsubsubsection{Transparent Encodings}{transparentencodings}
@@ -14001,6 +14007,10 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
     \item Correct the type signature of $\DiversifyHash{Orchard}$ in \crossref{abstracthashes}.
     \item Various rationale updates for \NUFive.
 } %nufive
+\sapling{
+    \item Describe the recommended way to encode a \Sapling\nufive{ or unified} \paymentAddress
+          as a QR code.
+} %sapling
     \item Add a definition for the \S{} symbol in \crossref{introduction}, before its first use.
     \item Remove specification of \memo contents, which will be in \cite{ZIP-302}.
     \item Remove support for building the \Sprout-only specification (\texttt{sprout.pdf}).

protocol/zcash.bib

@@ -1491,6 +1491,16 @@ Last revised February~5, 2018.}
    doi={10.1109/IEEESTD.2004.94612}
 }
 
+@misc{ISO2015,
+   author={ISO/IEC},
+   title={International {S}tandard {ISO/IEC} 18004:2015(E): {I}nformation {T}echnology --
+{A}utomatic identification and data capture techniques -- {QR} {C}ode bar code symbology specification.},
+   howpublished={Third edition},
+   date={2015-02-01},
+   url={https://raw.githubusercontent.com/yansikeim/QR-Code/master/ISO%20IEC%2018004%202015%20Standard.pdf},
+   urldate={2021-03-22}
+}
+
 @misc{Zcash-libsnark,
    presort={Zcash-libsnark},
    title={libsnark: {C}++ library for {zkSNARK} proofs (Zcash fork)},