Doc: fix missing merge on Release page

doc/md/dev/Release-Shaarli.md

@@ -64,6 +64,14 @@ git pull upstream master
 
 # If releasing a new minor version, create a release branch
 $ git checkout -b v0.x
+# Otherwise just use the existing one
+$ git checkout v0.x
+
+# Get the latest changes
+$ git merge master
+
+# Check that everything went fine:
+$ make test
 
 # Bump shaarli_version.php from dev to 0.x.0, **without the v**
 $ vim shaarli_version.php