dm: Update README to encourage conversion to driver model

Add a note to encourage people to convert drivers to use driver model.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index d072370..fe5cacb 100644
--- a/README
+++ b/README
@@ -5331,6 +5331,11 @@
 and make sure that your definition of IMAP_ADDR uses the same value
 as your U-Boot configuration in CONFIG_SYS_IMMR.
 
+Note that U-Boot now has a driver model, a unified model for drivers.
+If you are adding a new driver, plumb it into driver model. If there
+is no uclass available, you are encouraged to create one. See
+doc/driver-model.
+
 
 Configuring the Linux kernel:
 -----------------------------