chore: add AUTHORS file

AUTHORS

+# SPDX-FileCopyrightText: 2022 Georg-August-Universität Göttingen
+#
+# SPDX-License-Identifier: CC0-1.0
+#
+# This is the list of textgrid-clients's significant contributors.
+#
+# This does not necessarily list everyone who has contributed code.
+# To see the full list of contributors, see the revision history in
+# source control.
+Stefan Hynek
+Ubbo Veentjer