Add designate-manage path to tempest via a devstack script

So far, designate-manage was available to use in
designate-tempest-plugin tests, but its path was not well known.

This patch adds a devstack script that would set a tempest.conf setting
for the designate-manage path.

Change-Id: I26203522aaee9bb498bea9a1561eee65f6588244
diff --git a/devstack/README.rst b/devstack/README.rst
new file mode 100644
index 0000000..ee58314
--- /dev/null
+++ b/devstack/README.rst
@@ -0,0 +1,17 @@
+====================
+Enabling in Devstack
+====================
+
+1. Download DevStack::
+
+    git clone https://opendev.org/openstack/devstack.git
+    cd devstack
+
+2. Add this repo as an external repository::
+
+     > cat local.conf
+     [[local|localrc]]
+     enable_plugin designate-tempest-plugin https://opendev.org/openstack/designate-tempest-plugin
+
+3. run ``stack.sh``
+