Add NOTICE for erlang test grammar

Apache-2.0 on its own doesn't list the copyright holder since you are
not meant to edit the license text (unlike the MIT for example). So the
license file alone isn't enough, we also need the notice (recently
pushed upstream to tree-sitter-erlang). Also see the NOTICE in
tree-sitter-elixir which is also Apache-2.0 licensed.

test-grammars/erlang/NOTICE

@@ -0,0 +1,13 @@
+Copyright 2021 Michael Davis
+
+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.