doc: update preliminaries endianness

doc/preliminaries.tex

@@ -21,6 +21,15 @@ stored first. This distinction matters when serializing the internal state to
 produce the final digest --- MD5 uses little-endian, while SHA-256 and
 Whirlpool use big-endian.
 
+\[
+\text{Value}\; 0xD41D8CD9:
+\begin{cases}
+\text{Big-endian: } & \text{d4}\;\mid\;\text{1d}\;\mid\;\text{8c}\;\mid\;\text{d9}\\
+\text{Little-endian: } & \text{d9}\;\mid\;\text{8c}\;\mid\;\text{1d}\;\mid\;\text{d4}
+\end{cases}
+\]
+
+
 \vspace{1em}
 
 A \textbf{message} is the arbitrary-length input fed to a hash function. The