Add release script for briar-headless.jar

Moved from python-briar-wrapper: https://code.briarproject.org/briar/python-briar-wrapper/-/blob/0.0.3/tools/release-briar-headless.sh

python-briar-wrapper!9 (merged)

release-briar-headless.sh

+#!/usr/bin/env bash
+# Copyright (c) 2019 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+
+set -e -x
+
+cd briar
+
+./gradlew --configure-on-demand briar-headless:jar
+
+cd briar-headless/build/libs
+
+sha256sum briar-headless.jar > sha256sum.txt
+git log | head -n 1 > commit.txt
+
+echo "To sign, call 'for file in \$(ls); do gpg -b \"\$file\"; done'" > howto-sign.txt
+
+xdg-open .