README: add KMSXX_DEVICE & KMSXX_DRIVER

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
diff --git a/README.md b/README.md
index f0f3b97..feb43d7 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,8 @@
 --------------------------------- | -------------
 KMSXX_DISABLE_UNIVERSAL_PLANES    | Set to disable the use of universal planes
 KMSXX_DISABLE_ATOMIC              | Set to disable the use of atomic modesetting
+KMSXX_DEVICE                      | Path to the card device node to use
+KMSXX_DRIVER                      | Name of the driver to use. The format is either "drvname" or "drvname:idx"
 
 ## Python notes