Document the consensus rule that coinbase script length MUST be {2..100} bytes. fixes #589

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

protocol/protocol.tex

@@ -12620,6 +12620,7 @@ in \cite{ZIP-239}.
         length of $\heightBytes$ encoded as one byte, followed by $\heightBytes$ itself. This matches
         the encoding used by \Bitcoin in the implementation of \cite{BIP-34} (but the description here
         is to be considered normative).
+  \item A \coinbaseTransaction script \MUST have length in $\range{2}{100}$ bytes.
   \item %\consensuslink{bad-txns-premature-spend-of-coinbase}
         A \transaction \MUSTNOT spend a \transparentOutput of a \coinbaseTransaction
         from a \block less than 100 \blocks prior to the spend. Note that \transparentOutputs of
@@ -14624,6 +14625,8 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
     \item In \crossref{bctv}, note that the above checkpointing requirement
           mitigates the risks of not performing \BCTV \zkProof verification.
 } %sapling
+    \item Document the consensus rule that coinbase script length \MUST be
+          $\range{2}{100}$ bytes.
 \end{itemize}