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: NStephen Warren <swarren@wwwdotorg.org>

README

@@ -496,6 +496,13 @@ The following options need to be configured:
 		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