added support for power driver setting

Change-Id: I96323cd883f2a42878fa981a487e70e5122eaefe
diff --git a/README.rst b/README.rst
index 021b8c4..250ea82 100644
--- a/README.rst
+++ b/README.rst
@@ -112,6 +112,8 @@
             power_address: '192.168.10.10'
             power_user: bmc_user
             power_password: bmc_password
+            #Optional (for legacy HW)
+            power_driver: LAN
           distro_series: xenial
           hwe_kernel: hwe-16.04
       devices: