Clearly forward users to jupyterlab-contrib/jupyterlab-vim

[slavistan]

As per this comment and the general level of activity over at https://github.com/jupyterlab-contrib/jupyterlab-vim, this repository should either be frozen or, at least, the readme should clearly state that development is going on at jupyterlab-contrib/jupyterlab-vim.

Google directed me here and I only discovered jupyterlab-contrib/jupyterlab-vim by chance after I looked into #137 just for fun and saw the comment at the bottom of the thread. I think it would be very helpful to make a clear statement about the state of this repository.

What do you think?