Add citation for to Bitcoin Developer Reference for nbits.

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

protocol/protocol.tex

@@ -2151,7 +2151,8 @@ to the median time of the previous 11 blocks. \todo{has this changed?} A \fullno
 accept \blocks with headers more than two hours in the future according to its clock. \\ \hline
 
 4 & $\nBits$ & \type{uint32\_t} & An encoded version of the target threshold this \block's
-header hash must be less than or equal to, in the same nBits format used by \Bitcoin. \\ \hline
+header hash must be less than or equal to, in the same nBits format used by \Bitcoin.
+\cite{nbits} \\ \hline
 
 32 & $\nNonce$ & \type{char[32]} & An arbitrary field miners change to modify the
 header hash in order to produce a hash below the target threshold. \\ \hline

protocol/zcash.bib

@@ -166,6 +166,13 @@ L. Hern{\'a}ndez Encinas and C. S{\'a}nchez {\'A}vila},
    note={Accessed: \mbox{2016-08-08}}
 }
 
+@misc{nbits,
+   key={BitcoinTargetnBits},
+   title={Target n{B}its --- {B}itcoin {D}eveloper {R}eference},
+   howpublished={\url{https://bitcoin.org/en/developer-reference#target-nbits}},
+   note={Accessed: \mbox{2016-08-13}}
+}
+
 @book{std1363,
    author={IEEE Computer Society},
    publisher={IEEE},