Start RELEASE_NOTES.md and CHANGES.md sections for new release

CHANGES.md

@@ -13,6 +13,17 @@ Note that users who upgrade the simulator across versions, or who work directly
 
 This file is a best-effort approach to solving this issue; we will do our best but can guarantee that there will be things that fall through the cracks, unfortunately. If you, as a user, can suggest improvements to this file based on your experience, please contribute a patch or drop us a note on ns-developers mailing list.
 
+Changes from ns-3.40 to ns-3-dev
+--------------------------------
+
+### New API
+
+### Changes to existing API
+
+### Changes to build system
+
+### Changed behavior
+
 Changes from ns-3.39 to ns-3.40
 -------------------------------
 

RELEASE_NOTES.md

@@ -13,6 +13,15 @@ a [GitLab.com issue tracker](https://gitlab.com/nsnam/ns-3-dev/-/issues) number,
 and references prefixed by '!' refer to a
 [GitLab.com merge request](https://gitlab.com/nsnam/ns-3-dev/-/merge_requests) number.
 
+Release 3-dev
+-------------
+
+### Supported platforms
+
+### New user-visible features
+
+### Bugs fixed
+
 Release 3.40
 ------------