Align contributing doc with oslo's policy

Related to:
- https://review.opendev.org/#/c/723044/
- https://governance.openstack.org/tc/goals/selected/ussuri/project-ptl-and-contrib-docs.html

Change-Id: I6e02e68b689176a1a5e6adff0f32adf79ec473cf

CONTRIBUTING.rst

@@ -1,3 +1,8 @@
+If you would like to contribute to the development of oslo's libraries,
+first you must take a look to this page:
+
+   https://specs.openstack.org/openstack/oslo-specs/specs/policy/contributing.html
+
 If you would like to contribute to the development of OpenStack, you must
 follow the steps in this page: