Add Slack registration link to the contact section in Readme

[hauschke]

Is your feature request related to a problem? Please describe.
Most communication takes place in Slack currently. This should be reflected in the Readme.

Describe the solution you'd like
Add Slack as a means of communication to the Readme by adding a link to the registration form.

[chenejac]

@hauschke thanks for starting this discussion. At the moment, there is a link in the readme file to the https://vivo.lyrasis.org/contact/. There is a description of different approaches to reach the community (including registration to the Slack channel). I am wondering whether this is good enough?

[hauschke]

To engage devs, I think it might be worth to place this one piece of information a bit more prominent. Maybe some others can vote here, with thumbs up/thumbs down to this comment?