Suggest a better worker node.js impl in readme

[v1rtl]

• [x] Pull request has tests / docs demo, and is linted.
• [x] Description explains the issue / use-case resolved, and auto-closes the related issue(s) (https://help.github.com/articles/closing-issues-via-commit-messages/).

Since node-webworker is dead since 2011, I think it'd be more fair to suggest a newer, maintained worker threads implementation for Node.js, such as https://github.com/audreyt/node-webworker-threads in readme