Add script to release new version of Briar Headless

For this to work, you have to put this project (GTK) and Android's
project into one directory and call the Android's folder
"briar-android".

tools/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-android
+
+./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'"
+
+xdg-open .