start RELEASE_NOTES and CHANGES.html for next version

CHANGES.html

@@ -50,6 +50,24 @@ 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.</p>
 
+<hr>
+<h1>Changes from ns-3.20 to ns-3.21</h1>
+<h2>New API:</h2>
+<ul>
+</ul>
+
+<h2>Changes to existing API:</h2>
+<ul>
+</ul>
+
+<h2>Changes to build system:</h2>
+<ul>
+</ul>
+
+<h2>Changed behavior:</h2>
+<ul>
+</ul>
+
 <hr>
 <h1>Changes from ns-3.19 to ns-3.20</h1>
 <h2>New API:</h2>

RELEASE_NOTES

@@ -16,6 +16,26 @@ Availability
 ------------
 This release is not yet available.
 
+Supported platforms
+-------------------
+
+New user-visible features
+-------------------------
+
+Bugs fixed
+----------
+
+Known issues
+------------
+
+Release 3.20
+=============
+
+Availability
+------------
+This release is available from:
+http://www.nsnam.org/release/ns-allinone-3.20.tar.bz2
+
 Supported platforms
 -------------------
 - Fedora Core 20 (32/64 bit) with g++-4.8.2