website: Add a Slack invite link to the homepage

Change-Id: I71ee2f5a62a7318eb49c417d12b76428c50dcaed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/63251
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/index.html b/index.html
index 5c9f682..f0c1a76 100755
--- a/index.html
+++ b/index.html
@@ -21,6 +21,7 @@
   <a href="https://gem5.googlesource.com/public/gem5"><button type="button" class="btn btn-outline-primary">Source</button></a>
   <a href="https://gem5-review.googlesource.com/"><button type="button" class="btn btn-outline-primary">Code Review</button></a>
   <a href="https://arxiv.org/abs/2007.03152"><button type="button" class="btn btn-outline-primary">gem5-20+ paper</button></a>
+  <a href="https://join.slack.com/t/gem5-workspace/shared_invite/zt-1c8go4yjo-LNb7l~BZ0FagwmVxX08y9g"><button type="button" class="btn btn-outline-primary">Join our Slack</button></a>
   <a href="https://www.youtube.com/channel/UCCpCGEj_835WYmbB0g96lZw"><button type="button" class="btn btn-outline-primary">gem5 Youtube</button></a>
 
   <div class="testing-status">