Add release notes / changelog section for 2.13

CHANGELOG.txt

@@ -1,6 +1,12 @@
 Changelog
 =========
 
+2.13 (xx.xx.xxxx) - IN DEVELOPMENT
+~~~~~~~~~~~~~~~~~
+
+ * ...
+
+
 2.12 (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~
 

docs/releases/2.13.rst

@@ -0,0 +1,27 @@
+===========================================
+Wagtail 2.12 release notes - IN DEVELOPMENT
+===========================================
+
+.. contents::
+    :local:
+    :depth: 1
+
+
+What's new
+==========
+
+Other features
+~~~~~~~~~~~~~~
+
+ * ...
+
+
+Bug fixes
+~~~~~~~~~
+
+ * ...
+
+
+Upgrade considerations
+======================
+

docs/releases/index.rst

@@ -5,6 +5,7 @@ Release notes
    :maxdepth: 1
 
    upgrading
+   2.13
    2.12
    2.11.3
    2.11.2

docs/releases/upgrading.rst

@@ -141,3 +141,5 @@ The compatible versions of Django and Python for each Wagtail release are:
 +-------------------+------------------------------+-----------------------------+
 | 2.12              | 2.2, 3.0, 3.1                | 3.6, 3.7, 3.8, 3.9          |
 +-------------------+------------------------------+-----------------------------+
+| 2.13              | 2.2, 3.0, 3.1                | 3.6, 3.7, 3.8, 3.9          |
++-------------------+------------------------------+-----------------------------+