cgitrc.5.txt: document 'head-include'

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -84,6 +84,10 @@ footer::
 	verbatim at the bottom of all pages (i.e. it replaces the standard
 	"generated by..." message. Default value: none.
 
+head-include::
+	The content of the file specified with this option will be included
+	verbatim in the html HEAD section on all pages. Default value: none.
+
 header::
 	The content of the file specified with this option will be included
 	verbatim at the top of all pages. Default value: none.