docs: Add missing license header.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

docs/conf.py

@@ -1,4 +1,20 @@
 # -*- coding: utf-8 -*-
+# Copyright 2020 SkyWater PDK Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Configuration file for the Sphinx documentation builder.
 #