Create new release note sections for Wagtail 1.13

CHANGELOG.txt

@@ -1,6 +1,9 @@
 Changelog
 =========
 
+1.13 (xx.xx.xxxx) - IN DEVELOPMENT
+~~~~~~~~~~~~~~~~~
+
 1.12 LTS (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~~~~~
 

docs/releases/1.13.rst

@@ -0,0 +1,23 @@
+===========================================
+Wagtail 1.13 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
+   1.13
    1.12
    1.11.1
    1.11