Inform users about broken state and archive repository

It seems like this project is basically not ready for production and replaced by a faster and more reliable solution (go-mmproxy) which should be supported instead. It seems like Distributions like Debian or Ubuntu also packaged go-mmproxy already.

Problems at the moment:

* crashes randomly (memory corruption?)
* no actual development
* PRs are not merged
* better solutions exist which should be the target of development

I would therefore propose to:

* Add information in the README were to find the successor project go-mmproxy
* archieve the repository on github