Add reference to needed kernel version in manpage

Signed-off-by: Andi Kleen <ak@linux.intel.com>
diff --git a/mce-inject.8 b/mce-inject.8
index 6c98822..b6f5621 100644
--- a/mce-inject.8
+++ b/mce-inject.8
@@ -64,7 +64,8 @@
 mce-inject to run as root. /dev/mcelog has to exist. The kernels needs
 to have machine check injection (
 .I CONFIG_X86_MCE_INJECT
-) enabled.
+) enabled and if modular the mce-inject module loaded. This option is 
+available in Linux 2.6.31 or later.
 .SH NOTES
 The 
 .I mce-test