Add release note stubs for 2.5

CHANGELOG.txt

@@ -1,6 +1,10 @@
 Changelog
 =========
 
+2.5 (xx.xx.xxxx) - IN DEVELOPMENT
+~~~~~~~~~~~~~~~~
+
+
 2.4 (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~
 

docs/releases/2.5.rst

@@ -0,0 +1,23 @@
+==========================================
+Wagtail 2.5 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
+   2.5
    2.4
    2.3
    2.2.2