Add release note for dropping Django 2.0 support

CHANGELOG.txt

@@ -4,6 +4,7 @@ Changelog
 2.8 (xx.xx.xxxx) - IN DEVELOPMENT
 ~~~~~~~~~~~~~~~~
 
+ * Removed support for Django 2.0
  * Removed leftover Python 2.x compatibility code (Sergey Fedoseev)
  * Combine flake8 configurations (Sergey Fedoseev)
  * Improved diffing behavior for text fields (Aliosha Padovani)

docs/releases/2.8.rst

@@ -31,3 +31,8 @@ Bug fixes
 
 Upgrade considerations
 ======================
+
+Removed support for Django 2.0
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Django 2.0 is no longer supported as of this release; please upgrade to Django 2.1 or above before upgrading Wagtail.