Point out, that build/installkernel need a built world and that more

information can be found in the handbook.

Requested by:	sheldonh
Reviewed by:	sheldonh

README

@@ -13,14 +13,17 @@ commonly used one being ``world'', which rebuilds and installs
 everything in the FreeBSD system from the source tree except the
 kernel, the kernel-modules and the contents of /etc.  The
 ``buildkernel'' and ``installkernel'' targets build and install
-the kernel and the modules.  Please see the top of the Makefile
-in this directory for more information on the standard build targets
-and compile-time flags.
+the kernel and the modules (see below).  Please see the top of
+the Makefile in this directory for more information on the
+standard build targets and compile-time flags.
 
-Building a kernel with config(8) is a somewhat more involved process,
-documentation for which can be found at:
+Building a kernel is a somewhat more involved process, documentation
+for which can be found at:
    http://www.freebsd.org/handbook/kernelconfig.html
 And in the config(8) man page.
+Note: If you want to build and install the kernel with the
+``buildkernel'' and ``installkernel'' targets, you have to build
+world before.  More information is available in the handbook.
 
 The sample kernel configuration files reside in the sys/i386/conf
 sub-directory (assuming that you've installed the kernel sources), the