Follow the new PTI for document build

For compliance with the Project Testing Interface as
described in [1]:
[1] https://governance.openstack.org/tc/reference/project-testing-interface.html
For more detials information, please refer to [2]:
[2] http://lists.openstack.org/pipermail/openstack-dev/2017-December/125710.html

Change-Id: I648a78396a2e27f1d7197e89cddba73f8aa858aa
diff --git a/doc/requirements.txt b/doc/requirements.txt
new file mode 100644
index 0000000..b666029
--- /dev/null
+++ b/doc/requirements.txt
@@ -0,0 +1,6 @@
+# The order of packages is significant, because pip processes them in the order
+# of appearance. Changing the order has an impact on the overall integration
+# process, which may cause wedges in the gate later.
+openstackdocstheme>=1.17.0 # Apache-2.0
+reno>=2.5.0 # Apache-2.0
+sphinx>=1.6.2 # BSD