Add description of chinese-sisheng

* doc/emacs/mule.texi (Input Methods): Add description of
chinese-sisheng method for entering characters using pīnyīn.

doc/emacs/mule.texi

@@ -497,6 +497,10 @@ one of them selects that alternative.  The keys @kbd{C-f}, @kbd{C-b},
 do the highlighting in the buffer showing the possible characters,
 rather than in the echo area.
 
+  To enter characters according to the @dfn{pīnyīn} transliteration
+method instead, use the @code{chinese-sisheng} input method.  This is
+a composition based method, where e.g. @kbd{pi1} results in @samp{pī}.
+
   In Japanese input methods, first you input a whole word using
 phonetic spelling; then, after the word is in the buffer, Emacs
 converts it into one or more characters using a large dictionary.  One