unmodified_drivers: update README from http://wiki.xen.org/xenwiki/UnmodifiedDrivers

Add reference to the fact that these drivers are for "classic-Xen" kernels only
and do not work with PVops but point towards the PVHVM functionality in
mainstream.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>

unmodified_drivers/linux-2.6/README

@@ -1,10 +1,19 @@
-To build:
+These drivers provide paravirtualised drivers for pre-PVops Linux
+kernels which have the classic-Xen patches applied. They do not work
+with kernels which are not patched with the classic-Xen patches. In
+particular this includes all PVops kernels.
 
-1. ./mkbuildtree [<arch>]
+For 2.6.36 and newer you should use the "PVHVM" functionality which is
+available in the mainline kernel.
+
+To build do one of:
+
+   ./mkbuildtree [<arch>]
    NB. You can override paths to Xen sources and a (stub) XenLinux
        build tree via the XEN and XL environment variable.
 
-2. make -C /path/to/kernel/build M=$PWD modules
+- or -
+   make -C /path/to/kernel/build M=$PWD modules
    NB. This is your native kernel build tree (or a distro provided
        stub), not the XenLinux sources referred to in step 1.
    NB. If you are cross compiling, you need to set ARCH and