Include compile instructions for spank plugin example

doc/man/man8/spank.8

@@ -162,6 +162,11 @@ though system-wide default could be set using \fBSPANK\fR plugin arguments
 in the configuration file.
 .nf
 
+/*  
+ *   To compile:
+ *    gcc -shared -o renice.so renice.c
+ *
+ */
 #include <sys/types.h>
 #include <stdio.h>
 #include <unistd.h>