Update availability in RELEASE_NOTES.md

RELEASE_NOTES.md

@@ -13,8 +13,13 @@ 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
--------------
+Release 3.37
+------------
+
+### Availability
+
+This release is available from:
+<https://www.nsnam.org/release/ns-allinone-3.37.tar.bz2>
 
 ### Supported platforms