Add Apache 2.0 license (#60)

## Summary
- Adds the Apache 2.0 license file, matching the license used in
`ld-find-code-refs`.


Made with [Cursor](https://cursor.com)

Author: nieblara

LICENSE.txt

@@ -0,0 +1,13 @@
+Copyright 2019 Catamorphic, Co.
+
+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
+
+    http://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.