Update README for x970 memdiff values.
diff --git a/README b/README
index 5a8977c..c97f260 100644
--- a/README
+++ b/README
@@ -699,7 +699,8 @@ Sets card 0 cutoff temperature to 95 and card 1 to 105.
This setting will modify the memory speed whenever the GPU clock speed is
modified by --auto-gpu. In this example, it will set the memory speed to
be 125 Mhz lower than the GPU speed. This is useful for some cards like the
-6970 which normally don't allow a bigger clock speed difference.
+6970 which normally don't allow a bigger clock speed difference. The 6970 is
+known to only allow -125, while the 7970 only allows -150.
CHANGING SETTINGS: