Add release instructions and update release script

ci/manifest.sh

@@ -3,6 +3,15 @@ set -euxo pipefail
 IFS=$'\n\t'
 cd "$(dirname "$0")"/..
 
+bail() {
+    echo >&2 "error: $*"
+    exit 1
+}
+
+if [[ -z "${CI:-}" ]]; then
+    bail "this script is intended to call from release workflow on CI"
+fi
+
 git config user.name "Taiki Endo"
 git config user.email "te316e89@gmail.com"