[release] Add third-party license info to LICENSE file

This is the LICENSE file to be shipped with the extension.


Change-Id: Ia56c6aeb32c2fb192ca10be7cef18c61456f4f90
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/236749
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah