Add 2.8 section to changelog and release notes

CHANGELOG.txt

@@ -1,6 +1,11 @@
 Changelog
 =========
 
+2.8 (xx.xx.xxxx) - IN DEVELOPMENT
+~~~~~~~~~~~~~~~~
+
+ * ...
+
 2.7 LTS (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~~~~~
 

docs/releases/2.8.rst

@@ -0,0 +1,27 @@
+==========================================
+Wagtail 2.8 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
+   2.8
    2.7
    2.6.3
    2.6.2

docs/releases/upgrading.rst

@@ -131,4 +131,5 @@ The compatible versions of Django and Python for each Wagtail release are:
 +-------------------+------------------------------+-----------------------------+
 | 2.7 LTS           | 2.0, 2.1, 2.2                | 3.5, 3.6, 3.7, 3.8          |
 +-------------------+------------------------------+-----------------------------+
-
+| 2.8               | 2.0, 2.1, 2.2                | 3.5, 3.6, 3.7, 3.8          |
++-------------------+------------------------------+-----------------------------+