add instructions on including debugging from sources

Reference: /n/atom/patch/applied/yaccdbg
Date: Mon Feb 10 16:23:45 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/man/1/yacc	Mon Feb 10 16:23:28 2014
+++ /sys/man/1/yacc	Mon Feb 10 16:23:28 2014
@@ -39,6 +39,9 @@
 Create file
 .BR y.debug ,
 containing diagnostic messages.
+To incorporate them in the parser, compile it with preprocessor symbol
+.B yydebug
+defined.
 The amount of 
 diagnostic output from the parser is regulated by
 value