Add reference for DigiShield. (There doesn't appear to be any better reference than the source.)

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

protocol/protocol.tex

@@ -3295,7 +3295,7 @@ $\ToTarget(\nBitsField)$.
 
 \nsubsubsection{Difficulty adjustment} \label{diffadjustment}
 
-\Zcash uses a difficulty adjustment algorithm based on DigiShield v3/v4,
+\Zcash uses a difficulty adjustment algorithm based on DigiShield v3/v4 \cite{DigiByte-PoW},
 with simplifications and altered parameters, to adjust difficulty to target
 the desired 2.5-minute block time.
 Unlike \Bitcoin, the difficulty adjustment occurs after every block.

protocol/zcash.bib

@@ -416,6 +416,13 @@ Received \mbox{April 13,} 2011.}
    urldate={2016-09-02}
 }
 
+@misc{DigiByte-PoW,
+   author={DigiByte Core Developers},
+   title={DigiSpeed 4.0.0 source code, functions GetNextWorkRequiredV3/4 in src/main.cpp as of commit 178e134},
+   url={https://github.com/digibyte/digibyte/blob/178e1348a67d9624db328062397fde0de03fe388/src/main.cpp#L1587},
+   urldate={2017-01-20}
+}
+
 @misc{GitHub-mpc,
    author={Sean Bowe and Ariel Gabizon and Matthew Green},
    title={GitHub repository `\hairspace zcash/mpc'\hairspace: zk-SNARK parameter multi-party computation protocol},