Fix the way to get tempest.conf in README.rst

We're using oslo-config-generator to generate a config file other than
copy the tempest.config.sample. We should update the description in
qucikstart of README.rst.

Change-Id: I5c7c3f9041da3e5c3840b440bba6e614c23fdf8a
Closes-Bug: #1481635
diff --git a/README.rst b/README.rst
index d7063ba..431be7c 100644
--- a/README.rst
+++ b/README.rst
@@ -41,11 +41,13 @@
 will tell Tempest where to find the various OpenStack services and
 other testing behavior switches.
 
-The easiest way to create a configuration file is to copy the sample
-one in the ``etc/`` directory ::
+The easiest way to create a configuration file is to generate a sample
+in the ``etc/`` directory ::
 
     $> cd $TEMPEST_ROOT_DIR
-    $> cp etc/tempest.conf.sample etc/tempest.conf
+    $> oslo-config-generator --config-file \
+        tools/config/config-generator.tempest.conf \
+        --output-file etc/tempest.conf
 
 After that, open up the ``etc/tempest.conf`` file and edit the
 configuration variables to match valid data in your environment.