process: add new section History

History are used to make easy users to follow the
evolution of the spec.

The spec can differ with the time, and the design can be updated
it should be important for users who use the spec has a documentation
to see those changes without to use a third tool.

The initial proposal is to add an entry for each update provided to
the spec (even fixing a typo) but we can restrain it only for changes
in the design or new contributor refered...

Change-Id: I13b8e09d5d7848ee4657214a86d2b48bf1b05e2a

specs/liberty-template.rst

@@ -373,3 +373,20 @@ references are unavailable. Examples of what you could include are:
   EC2 docs)
 
 * Anything else you feel it is worthwhile to refer to
+
+
+History
+=======
+
+Optional section for liberty intended to be used each time the spec
+is updated to describe new design, API or any database schema
+updated. Useful to let reader understand what's happened along the
+time.
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Liberty
+     - Introduced

tests/test_titles.py

@@ -17,6 +17,12 @@ import docutils.core
 import testtools
 
 
+# Used for new sections introduced during a release.
+# - "History" introduced in Liberty should be
+# mandatory for M.
+OPTIONAL_SECTIONS = ("History",)
+
+
 class TestTitles(testtools.TestCase):
     def _get_title(self, section_tree):
         section = {
@@ -40,7 +46,8 @@ class TestTitles(testtools.TestCase):
         return titles
 
     def _check_titles(self, filename, expect, actual):
-        missing_sections = [x for x in expect.keys() if x not in actual.keys()]
+        missing_sections = [x for x in expect.keys() if (
+            x not in actual.keys()) and (x not in OPTIONAL_SECTIONS)]
         extra_sections = [x for x in actual.keys() if x not in expect.keys()]
 
         msgs = []