add a note about non ATA-7 drives with new kernels

Author: evgeni

README

@@ -20,6 +20,11 @@ for more options, please read man hdapsd
 ----------------
 Since kernel 2.6.28 you don't need to patch your kernel, as support for
 IDLE_IMMEDIATE is present in mainline.
+ NOTE: The new interface only allows IDLE_IMMEDIATE for drives that
+       announce to be ATA-7 conform. But threre are also drives that
+       support ATA-6 only but do IDLE_IMMEDIATE fine. For those you
+       need to force the interface with:
+       echo -1 > /sys/block/$DISK/device/unload_heads
 
 For kernels <2.6.28, please have a look at
 http://www.thinkwiki.org/wiki/HDAPS#Kernel_patch