Add installation procedure of cookiecutter to doc

Currently, in Usage, there is no installation procedure of cookiecutter.
This patch set adds installation procedure of cookiecutter to README.rst

Change-Id: Ief3a1fdab945c1df834b219fadadf8d2b5d824a4
diff --git a/doc/source/plugin.rst b/doc/source/plugin.rst
index a41038c..29653a6 100644
--- a/doc/source/plugin.rst
+++ b/doc/source/plugin.rst
@@ -26,7 +26,7 @@
 In order to create the basic structure with base classes and test directories
 you can use the tempest-plugin-cookiecutter project::
 
-  > cookiecutter https://git.openstack.org/openstack/tempest-plugin-cookiecutter
+  > pip install -U cookiecutter && cookiecutter https://git.openstack.org/openstack/tempest-plugin-cookiecutter
 
   Cloning into 'tempest-plugin-cookiecutter'...
   remote: Counting objects: 17, done.