Add an obvious deprecation statement to README

[tansongyang]

I see from this issue that you are not supporting this library anymore. From my perspective, the current warning does not make this explicit:

For a more complete (and more active) functional framework including most of the monads listed above, please check my other project: Language-Ext

Consider adding something like this to the top of the README:

This project has been superseded by Language-Ext. You should use that instead. I will review and accept PRs for this project, but that's it. There will be no new development.

If you accept this wording, I can PR.