README: re-add the chat link

[sandrock]

Hello,

Recently, the link to the community chat has been removed from the README file.

As it may have been an an error, I suggest to put it back.

There are from time to time new users asking questions there and I think it is best to keep it alive.

If you agree, this PR contains the link to the chat.

Not being the maintainer of the project any more, I can set a new admin in the chat.