scripts/do-release: add .sh suffix and add license ID

Signed-off-by: Eric Biggers <ebiggers@google.com>
diff --git a/scripts/do-release b/scripts/do-release.sh
similarity index 93%
rename from scripts/do-release
rename to scripts/do-release.sh
index d129bf5..2a02c53 100755
--- a/scripts/do-release
+++ b/scripts/do-release.sh
@@ -1,4 +1,5 @@
 #!/bin/bash
+# SPDX-License-Identifier: GPL-2.0+
 
 set -e -u -o pipefail