Add a reference to the sha module, noting that SHA is considered more

secure.  Suggested by Erno Kuusela <erno@iki.fi>.

Doc/lib/libmd5.tex

@@ -70,3 +70,10 @@ Return a copy (``clone'') of the md5 object.  This can be used to
 efficiently compute the digests of strings that share a common initial
 substring.
 \end{methoddesc}
+
+
+\begin{seealso}
+  \seemodule{sha}{Similar module implementing the Secure Hash
+                  Algorithm (SHA).  The SHA algorithm is considered a
+                  more secure hash.}
+\end{seealso}