README: document the requirements for CONFIG_SYS_HZ

CONFIG_SYS_HZ must be 1000, and get_timer() must therefore return ms.
Document this.

README text provided by Tom Rini.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
diff --git a/README b/README
index 1d51828..5701016 100644
--- a/README
+++ b/README
@@ -496,6 +496,13 @@
 		exists, unlike the similar options in the Linux kernel. Do not
 		set these options unless they apply!
 
+- CPU timer options:
+		CONFIG_SYS_HZ
+
+		The frequency of the timer returned by get_timer().
+		get_timer() must operate in milliseconds and this CONFIG
+		option must be set to 1000.
+
 - Linux Kernel Interface:
 		CONFIG_CLOCKS_IN_MHZ