diff --git a/INSTALL b/INSTALL
index 26bd704..749b3b5 100644
--- a/INSTALL
+++ b/INSTALL
@@ -15,6 +15,10 @@ Building:
 	download and compile them yourself from http://flex.sourceforge.net/
 	and http://gcc.gnu.org.
 
+	For building optional documentation you need sgmlformat-1.7.tar.gz
+	from ftp://ftp1.freebsd.org:/pub/FreeBSD/distfiles and docbook which
+	will be probably already included in your distro.
+
 	SCons is replacement for configure and make utility; there is no
 	separate configuration step. Building with SCons is very similar
 	to using make. Run 'scons' to compile and 'scons install' to