message: fix(readme): update slack button
diff: @@ -15,7 +15,8 @@ Best way to quickly integrate <a href="https://getbootstrap.com/">Bootstrap 3</a
  <a target="_blank" href="https://opencollective.com/ngx-bootstrap"><img src="https://opencollective.com/ngx-bootstrap/tiers/backer/badge.svg?label=backer&color=brightgreen" /></a>
  <br/>
  -<a href="https://join.slack.com/t/ngx-home/shared_invite/enQtNTExMTY5MzcwMTM0LWVjZGU2MjI4MTVhMGVlMTc2OWRiMzA0NzBhNDU5YzQ0MDM3MWI5NzJjZTUzNzIxZmNjYmFlMjU2MzE0YmY0NWY"><img src="https://thehomeofthefuture.files.wordpress.com/2018/12/Joinslack.png" width="20%" alt="slack" ></a>
  +<a href="https://join.slack.com/t/ngx-home/shared_invite/enQtNTExMTY5MzcwMTM0LWVjZGU2MjI4MTVhMGVlMTc2OWRiMzA0NzBhNDU5YzQ0MDM3MWI5NzJjZTUzNzIxZmNjYmFlMjU2MzE0YmY0NWY">
  +<img src="https://cdn.brandfolder.io/5H442O3W/at/pl546j-7le8zk-btwjnu/Slack_RGB.png?width=1200&height=150" alt="slack" ></a>
  </p>
  