Add License to the README file

Summary:
Fixing a checkup from https://www.internalfb.com/intern/opensource/github/repo/1868531010203915/checkup

The README file does not mention the license explicitly.
Error: File README.md doesn't contain license

Reviewed By: angelayi, cccclai

Differential Revision: D47817412

fbshipit-source-id: 3d2888f4e759045ce6765f71429d63a180fcdbcb

Author: mergennachin

README.md

@@ -42,3 +42,6 @@ Compared to the legacy Lite Interpreter, there are some major benefits:
     - [EXIR Spec](/docs/website/docs/ir_spec/00_exir.md)
     - [Delegate to a backend](/docs/website/docs/tutorials/backend_delegate.md)
 - [Executorch Google Colab](https://colab.research.google.com/drive/1oJBt3fj_Tr3FE7L9RdUgSKK9XzJfUv4F#scrollTo=fC4CB3kFhHPJ))
+
+## License
+ExecuTorch is BSD licensed, as found in the LICENSE file.