Update RELEASE.md

Author: Éric Lemoine

RELEASE.md

@@ -7,8 +7,12 @@ Add a new section to the [NEWS](NEWS) file, listing all the changes associated w
 Change the version number in the [Version.config](Version.config) and
 [pgsql/expected/pointcloud.out](pgsql/expected/pointcloud.out) files.
 
-Create a PR with these changes. When the PR is merged create a tag for the new release and push
-it to GitHub:
+Update the value of `UPGRADABLE` in [pgsql/Makefile.in](pgsql/Makefile.in). This variable defines
+the versions from which a database can be upgraded to the new Pointcloud version.
+
+Create a PR with these changes.
+
+When the PR is merged create a tag for the new release and push it to GitHub:
 
 ```bash
 $ git tag -a vx.y.z -m 'version x.y.z'