Put back the README.md file in the npm package

[piaskowyk]

Summary

This PR adds the README.md file back to the NPM package. After migration to a monorepo, the README.md file's location was changed to the parent directory. Unfortunately, you can't include a file from the parent directory in an npm package, and symlinks don't work for this either.

Test plan

run npm pack and check if README.md is inside