Fix broken link in CONTRIBUTING

Signed-off-by: Sebastiaan van Stijn <[email protected]>

CONTRIBUTING.md

@@ -88,7 +88,7 @@ use for simple changes](https://docs.docker.com/opensource/workflow/make-a-contr
   <tr>
     <td>Community Slack</td>
     <td>
-      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://community.docker.com/registrations/groups/4316" target="_blank">with this link</a>.
+      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://dockr.ly/slack" target="_blank">with this link</a>.
     </td>
   </tr>
   <tr>
@@ -362,4 +362,4 @@ The rules:
 If you are having trouble getting into the mood of idiomatic Go, we recommend
 reading through [Effective Go](https://golang.org/doc/effective_go.html). The
 [Go Blog](https://blog.golang.org) is also a great resource. Drinking the
-kool-aid is a lot easier than going thirsty.
+kool-aid is a lot easier than going thirsty.