[ADD] orm/changelog: Add a changelog page for the ORM changes

Introduce changelog of the ORM by version, link to first change documented

closes odoo/documentation#1508

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

content/developer/reference/backend/orm.rst

@@ -1,9 +1,16 @@
+:show-content:
+
 .. _reference/orm:
 
 =======
 ORM API
 =======
 
+.. toctree::
+   :titlesonly:
+
+   orm/changelog
+
 .. automodule:: odoo.models
 
 .. _reference/orm/models:

content/developer/reference/backend/orm/changelog.rst

@@ -0,0 +1,13 @@
+.. _reference/orm/changelog:
+
+=========
+Changelog
+=========
+
+Odoo Online version 15.2
+========================
+
+- Specific index types on fields:  With `#83274 <https://github.com/odoo/odoo/pull/83274>`_ and
+  `#83015 <https://github.com/odoo/odoo/pull/83015>`_, developers can now define what type of
+  indexes can be used on fields by PostgreSQL. See the :ref:`index property <reference/fields>` of
+  `odoo.fields.Field`.