added note about the low precedence of shell to config.def.h

config.def.h

@@ -7,6 +7,15 @@
  */
 static char font[] = "Liberation Mono:pixelsize=12:antialias=false:autohint=false";
 static int borderpx = 2;
+
+/*
+ * What program is execed by st depends of these precedence rules:
+ * 1: program passed with -e
+ * 2: utmp option
+ * 3: SHELL environment variable
+ * 4: value of shell in /etc/passwd
+ * 5: value of shell in config.h
+ */
 static char shell[] = "/bin/sh";
 static char *utmp = NULL;
 static char stty_args[] = "stty raw -echo -iexten echonl";