Add Oslo feature freeze to release schedule

Oslo observes feature freeze one week prior to the final non-client
library release in order to allow projects to consume new features
before their feature freezes.[1]

Change-Id: Ic4652d859c4118fc0a1b994193609c2fc1789ff3
1: http://specs.openstack.org/openstack/oslo-specs/specs/policy/feature-freeze.html

doc/source/stein/schedule.rst

@@ -343,3 +343,13 @@ Cinder Feature Proposal Freeze
 All new Cinder features must be proposed and substantially complete with unit tests
 by 07 Mar 2019 (23:59 UTC).
 
+Oslo
+----
+
+.. _s-oslo-feature-freeze:
+
+Oslo Feature Freeze
+^^^^^^^^^^^^^^^^^^^
+
+All new Oslo features must be proposed and substantially complete, with unit
+tests by the end of the week.

doc/source/stein/schedule.yaml

@@ -111,6 +111,7 @@ cycle:
   start: '2019-02-18'
   project-specific:
     - s-manila-fp-freeze
+    - s-oslo-feature-freeze
 - end: '2019-03-01'
   name: R-6
   start: '2019-02-25'